The invention relates to a method and a device for pesticide detection using cholinesterase. Particularly,-the invention relates to a new pesticide detection method based on magnetic nanoparticles, on which cholinesterase (acetylcholinesterase, ACE) is immobilized, wherein the magnetic nanoparticles serve as carriers to collect the cholinesterase in a solution. Furthermore, the invention relates to a detector for pesticide detection that employs the above method. BACKGROUND OF THE INVENTION On average, 2.5 million tons of pesticides are consumed each year all over the world and most of them are applied to crops. Unfortunately, over 95% of pesticides reach air, water, and soil through rainfall and sunlight, which cause serious pollution. On the other hand, pesticides remained in crops may also cause severe food poisoning accidents. The World Health Organization (WHO) estimates that there are one million pesticide poisoning cases and 20,000 deaths every year globally. Hence, it is urgent and important to develop a portable household pesticide sensor. Organochlorines, organophosphates and carbamates were three major pesticides in the 1960s. However, it was discovered that most of organochlorines are persistent organic pollutants which cause potential hazards when they are released into the environment. For example, DDT, which was widely used to control pests in the mid 20th century, which is shown in researches that it also acc&ulates in aquatic food chains and has been banned in a lot of countries. Currently, organophosphates and carbamates have largely replaced organochlorines and the two pesticides share around 70% of the pesticide market. Both of organophosphates and carbamates function through inhibiting the activity of acetylcholinesterase (AChE) and thus causing excessive acetylcholine (ACh, the substrate of AChE) to accumulate in the synaptic cleft. The excessive acetylcholine causes neuromuscular paralysis (i.e. interminable muscle contractions) throughout the entire body, leading to death by asphyxiation. Based on the inhibitory effect of organophosphates and carbarnates on AChE activity, some pesticide sensors have been developed. Basically, their detection methods can be classified into two categories. One is by adding 5,5'-dithiobis-(2-nitrobenzoic acid) (DTNB), known as Ellrnan's reagent and detecting via a spectrometer. DTNB reacts with thiocholine (TCh) which is the a hydrolysis product of acetylthiocholine (ATCh, ACh is replaced by ATCh to provide a disulfide bond) to give lm, namely 2-nitro-5-thiobenzoic acid, which is a yellow product with the maximum absorption at 412 nrn, as shown in Fig. 1. The other detection method comprises pH measurement and electrochemical method, etc., as shown in Figure 2. Compared to the conventional methods requiring professionals and only useful in analytical chemistry laboratories (chromatography and coupled chromatography-spectrometry: GC-MS, HPLC-MS), AChE detection system has the potential to complement or replace the conventional methods by simplifying or eliminating sample preparation protocols and making in situ testing easier and faster as well as significantly decreasing in the cost per analysis. However, the poor reliability and short lifespan of AChE detection system still restricts its wider application. Its reliability can be enhanced by increasing the signal to noise ratio (SN, and the short lifespan of AChE resulting from the irreversible or partially reversible inhibition reaction between AChE and the pesticide can be easily overcome by replacing with fresh AChE. The traditional way to increase S/N is via the immobilization of AChE on the surface of an electrode with high electron transfer. Various matrix materials have beenemployed, such as Prussian blue,7 cobalt phthalocyanine (CoPc), Poly(amido-arnine) (PAMAM), polyaniline, Au nanoparticles (AuNPs), muti-walled carbon nanotubes (MWNTs), A1203, Si02, ZnO and so on. A low detection limit formethyl paraoxon has been reduced to 6x lo-'' M in amperometric biosensor based on AChE immobilized on AuNPs and silk fibroin modified Pt electrode. The limitation of this amperometric sensor in practicable application is its short lifespan in pesticide detection. Although the inhibited AChE could be partially reactivated by oximes, such as prolidoxime iodide, to retain the activity of AChE, the ACE-immobilized layer on electrode should still be replaced frequently. The replacement of AChE layers needs careful handling; otherwise the ACE layer will lose contact with electrode so as to affect the measurement. To avoid the AChE reactivation which is time-consuming and the replacement of ACE layer which is labor-consuming, herein, we invented a new pesticide detection method based on AChE-immobilized water-soluble magnetic nanoparticles. In this system, AChE is still the signal source and magnetic nanoparticles serve as an AChE carrier to increase S/N by collecting AChE using magnetic field. When the particle size of the magnetic nanoparticles is smaller than 20 nrn, the magnetic nanoparticles are superparamagnetic, which means that if the magnetic field is removed; these water-soluble magnetic nanoparticles can be washed away by water. By this way, the inhibited AChE can be easily replaced after detection. Because fiesh AChE is used every time, no reactivation is nedded, and the drawback of short lifespan of AChE can be avoided. CONTENS OF THE INVENTION The invention relates to a method and a device using acetylcholinesteraseirnmobilized with magnetic nanoparticles for detecting pesticide. In particular, the invention relates to the following aspects: 1. A method for pesticide detection, comprising the steps of: (a) providing a sample to be detected; (b) preparing magnetic nanoparticles, and contacting cholinesterase with the magnetic nanoparticles to obtain cholinesterase-irnmbolilized magnetic nanoparticles; (c) adding the cholinesterase-immbolilized magnetic nanoparticles obtained in step (b) to the sample to be detected of step (a), and incubating; (d) collecting the magnetic nanoparticles by a magnetic field; and (e) removing the magnetic field, dispersing the collected magnetic nanoparticles in an enzymatic activity detection system, dectecting the enzymatic activity of cholinesterase, and determining the presence or the content of the pesticide based on the enzymatic activity. 2. The method according to aspect 1, wherein the pesticide is a chemically synthesized pesticide. 3. The method according to aspect 1, wherein the pesticide is selected fiom the group consisting of organophosphates pesticides, carbarnates pesticides, organic nitrogens pesticides, pyrethroids pesticides, organochlorines pesticides and organoflurines pesticides. 4. The method according to aspect 3, wherein the organophosphates pesticide is selected from the group consisting of: phosphates, monothiophosphates, dithiophosphates, phosphonates, phosphoamides, thiophosphoamides, and pyrophosphates. 5. The method according to aspect 3, wherein the organophosphates pesticide is slected fiom: glyphosate, dimethoate and Malaoxon. 6. The method according to aspect 3, wherein the carbamates pesticide is selected fiom the group consisting of: N-methylcarbarnates and dimethylcarbamates. 7. The method according to aspect 3, wherein the carbarnates pesticide is selected fiom Carbaryl and Isoprocarb. 8. The method according to any of aspects 1 to 7, wherein the magnetic nanoparticles is prepared by a method selected from the group consisting of: a wet chemistry method, a chemistry vapor method and a physical method. 9. The method according to aspect 8, wherein the wet chemistry method is selected from a precipitation method, a sol-gel method, a microemulsion and inverse microemulsion method, a hydrothermal method and a polyol reduction method; the chemistry vapor method is selected from a chemical vapor deposition, a chemical vapor condensation and a plasma evaporation method; and the physical process is selected from an evaporation condensation method and a magnetron sputtering method. 10. The method according to any of aspects 1 to 9, wherein the magnetic nanoparticle is selected from the magnetic nanoparticles of alloy, ferrite and / or intermetallic compound. 11. The method according to any of aspects 1 to 10, wherein the magnetic nanoparticle is magnetic nanoparticles containing iron, cobalt, nickel or the alloy thereof, preferably Fe304 nanoparticles. 12. The method according to any of aspects 1 to 11, wherein the surface of the magnetic nanoparticle comprises polar group, preferably citrate or hydroxyl group. 13. The method according to any of aspects 1 to 12, wherein the cholinesterase is selected from the group consisting of: acetylcholinesterase, propionylcholinesterase and butyrylcholinesterase. 14. The method according to aspect 13, wherein the acetylcholinesterase is obtained from a source selected from the group consisting of: microorganism, plants and animals, preferably electric eels or electric skates. 15. The method according to any of aspects 1 to 14, further comprising a step of calibrating the magnetic nanoparticles, preferably by a standard curve, prior to step (b). 16. The method according to any of aspects 1 to 15, further comprising a step of calibrating concentration of cholinesterase, preferably by a standard curve, prior to step (b). 17. The method according to any of aspects 1 to 16, further comprising a step of washing the magnetic nanoparticles, preferably using deionized water, for 1-5 times, more preferably for 4 times, 'prior to step (b). 18. The method according to any of aspects 1 to 17, wherein the immobilization of cholinesterase onto magnetic nanoparticles in step (b) is carried out by a method selected fiom the group consisting of: adsorption method, chemical crosslinking method, covalent linking method, physical embedding method, electrochemical polymerization method and molecular self-assembly. 19. The method according to any of aspects 1 to 18, wherein the immobilization of cholinesterase onto magnetic nanoparticles in step (b) is carried out under a condition of pH 6-8, preferably pH 7. 20. The method according to any of aspects 1 to 19, wherein during the immobilization of cholinesterase onto magnetic nanoparticles in step (b), cholinesterase and the magnetic nanoparticles are added in a ratio of 20-330: 1 Ulg, preferably 125-320: 1 Ulg, more preferably 250-3 10: 1 Ulg, more preferably 280: 1 Ulg. 21. The method according to any of aspects 1 to 20, wherein the immobilization of cholinesterase onto magnetic nanoparticles in step (b) is carried out as follows: (A) dispersing the magnetic nanoparticles in deionized water or a phophate buffered solution; (B) dissolving cholinesterase in deionized water or a phophate buffered solution containing bovine serum albumin (BSA); and (C) contacting the magnetic nanoparticles solution with the cholinesterase solution. 22. The method according to any of aspects 1 to 21, wherein the immobilization of cholinesterase onto magnetic nanoparticles in step (b) is carried out as follows: (a) dispersing the magnetic nanoparticles in deionized water; (b) dissolving cholinesterase in a phophate buffered solution containing bovine serum albumin (BSA); and (c) contacting the magnetic nanoparticles solution with the cholinesterase solution. 23. The method according to any of aspects 2 1 to 22, wherein the concentration of the magnetic nanoparticles solution is 0.1-1 0 g/L,pr eferably 0.5-5 g/L, more preferably 1-3 g/L,m ore preferably approximately 2 g L . 24. The method according to any of aspects 21 to 22, wherein the concentration of cholinesterase solution is 0.1-10 Ulml, preferably 0.5-2 Ulml, more preferably 1 Ulml. 25. The method according to any of aspects 21 to 22, wherein the concentration of bovine serum albumin in phosphate buffered solution is 0.1-5 g/L, preferably 0.5-2 g/L, more preferably 0.8-1.2 g/L,m ore preferably 1 g/L. 26. The method according to any of aspects 1 to 25, wherein 0.05-1 ml, preferably 0.1-0.5 ml, more preferably 0.25 ml of cholinesterase-immbolilized magnetic nanoparticles solution is added into the sample to be detected in step (c). 27. The method according to any of aspects 1 to 26, wherein, in step (c), the sample to be detected and the cholinesterase-immbolilized magnetic nanoparticles are incubated at a temperature of 0-60°C, preferably 20-50°C, more preferably 40°C for 2 minutes - 2 hours, preferably 20 minutes - 1 hour, more preferably 40 minutes. 28. The method according to any of aspects 1 to 27, wherein the magnetic nanoparticles are collected by a magnet coated with an outer layer in step (d). 29. The method according to aspect 28, wherein the diameter of the magnet is approximately 2-10 mm, preferably 3-8 mm, more preferably 4 mm. 30. The method according to any of aspects 1 to 29, wherein the step (d) is performed for 1-10 minutes, preferably 1-5 minutes, more preferably 100 seconds. 31. The method according to any of aspects 1 to 30, wherein at least 60%, preferably at least 80%, more preferably at least 90%, more preferably substantially 100% of the magnetic nanoparticles are colleced in step (d). 32. The method according to any of aspects 1 to 3 1, wherein the enzymatic activity detection system in step (e) has a pH value of 2- 1 1, preferably 6- 10, more preferably 7-9. 33. The method according to any of aspects 1 to 32, wherein the detection of the enzymatic activity of cholinesterase in step (e) is performed by a pH value measurement method, an electrochemistry method or a spectrophotometry method, preferably by the spectrophotometry method. 34. The method according to aspect 33, wherein, in step (e), 5,5'dithiobis-(2-nitrobenzoic acid) (DTNB) and thiocholine (TCh) are added for detecting the enzymatic activity of cholinesterase by the spectrophotometry method. 35. The method according to aspect 34, wherein the spectrophotometry method is used to determine the speed of absorbency change; the higher speed of absorbency change indicates a higher 7 enzymatic activity, whereas the lower speed of absorbency change indicates a lower enzymatic activity. 36. The method according to any of aspects 1 to 35, wherein the higher enzymatic activity determined in step (e) indicates a smaller amount of pesticide contained in the sample to be detected, whereas the lower enzymatic activity determined in step (e) indicates a larger amount of pesticide contained in the sample to be detected. 37. A portable detector for pesticide detection, comprising: a power supply system, a storage system, a control system, and a determination and analysis system. 38. The detector according to aspect 37, wherein the power supply system is a battery (I), which is used for supply electricity to other systems of the detector. 39. The detector according to any of aspects 37-38, wherein the storage system comprises a DTNB container (5), a thiocholine container (6), a magnetic nanoparticle solution container (13), and a magnet device (14) provided with a magnet for collecting and transferring the magnetic nanoparticles. 40. The detector according to aspect 39, wherein the magnet device (14) comprises a casing (14a) and an inside magnet (14b), wherein the inside magnet (14b) is removable from the casing (14a). 41. The detector according to aspect 40, wherein the casing (14a) is made of glass or polytetrafluoroethylene. 42. The detector according to any of aspects 37-41, wherein the control system comprises a button (3) connected to the DTNB container (5) for controlling the addition of DTNB, a button (4) connected to the thiocholine container (6) for controlling the addition of cholinesterase, and a button (12) for controlling the release of a cholinesterase-imrnbolilized magnetic nanoparticles solution. 43. The detector according to any of aspects 37-42, wherein the determination and analysis system comprises a light emitting diode (8) that provides the wavelength required by the spectrum detection; a chip (2) connected to the light emitting diode (8) for controlling the light emitting diode; a cuvette (9) that conntected to the DTNB container (5) through the button (3), and connected to thiocholine container (6) through the button (4), and can be inserted in to the cuvette (9) of the magnet device(l4); a photoelectric conversion device (10) for converting the collected optical signal to electrical signal; a liquid crystal screen (7) for displaying the detected results; and a data processing chip (1 1) connected to the photoelectric conversion device (1 0) and the liquid crystal screen (7), repectively. 44. The detector according to any of aspects 37-43, wherein the detector is pen-like. 45. A method of pesticide detection using a detector as defined in any of aspects 37 to 44, comprising the steps of: (a) pressing button (12), so that a solution of cholinesterase-immbolilized magnetic nanoparticles is contacted with a sample to be detected; (b) collecting the magnetic nanoparticles in the sample by a magnet device (14), inserting the magnet device (14) into cuvette(9), and then removing the inside magnet (14b) from the magnet device(l4); (c) pressing button (3), adding DTNB and the cholinesterase-immbolilized magnetic nanoparticles on the casing (14a) of the magnet device(l4) into the the cuvette (9); (d) connecting to the electric supply, and pressing button (4) to add acetylthiocholine into the cuvette after the reading on the liquid crystal display screen becomes stable; and (e) determining the concentration of the pesticide in the sample according to the color change speed of the cuvette by a determination and analysis system, and displaying on the iquid crystal display screen (7). 46. The method according to aspect 45, wherein step (a) is carried out for 1-5 minutes, preferably 3 minutes. 47. The method according to aspect 45, wherein the collecting in step (b) is carried out for 10-60 seconds, preferably 30 seconds. 48. A use of a detector according to any of aspects 37 to 44 for pesticide detection. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 shows the chemical equation for the hydrolysis of ATCh and for the reaction between TCh and DTNB, as well as the UV-vis spectrum for TNB. Fig. 2 shows an exemplary method for pesticide detection by acetylcholinesteraseimmoblized magnetic nanoparticles. Fig. 3 shows the recollection of the magnetic nanoparticles by a magnet. Fig. 4(a) shows a XRD pattern of the magnetic nanoparticles precipitated at 100°C with Fe304 as reference; Fig. 4 (b) shows the TG curve of magnetic nanoparticle powders. Fig. 5(a) shows the UV-vis spectrum for magnetic nanoparticles; Fig. 5(b) shows the standard curve for the concentration of the magnetic nanoparticles and the absorbance thereof at 412 nrn. Fig. 6(a) shows the UV-vis spectrum of the reaction scanned at 412 nrn; Fig. 6(b) shows the standard curve for the concentration of acetylcholinesterase and the slope at the beginning of the reaction. Fig. 7(a) shows the effect of the times of washing magnetic nanoparticles on the amount of acetylcholinesterase immobilized on MNPs; Fig. 7(b) shows the influence of the pH value of the PBS buffer solution on the amount of immobilized acetylcholinesterase. Fig. 8(a) shows the influence of the volume of the acetylcholinesterase added into the PBS @H=7.4) on the response time; Fig. 8(b) shows the influence of the pH value of the detection system with 125 pl of acetylcholinesterase (1 Ulml) solution added on the activity of acetylcholinesterase. Fig. 9 shows an analysis of the stability of the free and immobilized acetylcholinesterases. Fig. 10 shows an analysis of the stability of DTNB (stored at an ambient temperature for 40 Fig. 11 shows the responses of acetylcholinesterase to the following different pesticides: dimethoate, glyphosate, Carbaryl, Isoprocarb and Malaoxon. Fig. 12 shows the inhibition curve of acetylcholinesterase-immobilized magnetic nanoparticles by Carbaryl. Fig. 13 shows the relaitionship between the amount of added acetylcholinesterase and the amount of the actually immobilized enzyme (n=3) when acetylcholinesterase is immobilized in a certain amount of magnetic nanoparticles solution. Fig. 14 shows the relationship between the amount of the immobilized acetylcholinesterase and the pH value (n=3). Fig. 15 shows the influence of the pH value of acetylcholinesterase (in a free state, without being immobilized on the surface of magnetic nanoparticles) at detection on activity: Fig. 15(a) 10 shows the absorbance change at 412 nrn at different pH values, wherein the slope of UV-vis spectrometry spectrum curve changing with the pH value reflects the enzymatic activity changing with the pH value; Fig. 15(b) shows the enzymatic activity curve at a pH value of 2-1 1 (wherein the enzymatic activity is the result obtained by normalization based on the point (pH=9) having the highest enzymatic activity). Fig. 16 shows the addition of the polymer V.S. the amount of the immobilized acetylcholinesterase (n=3) during immobilizaion. Fig. 17 (left) shows the inhibition effect of the addition of PDDA on the activity of acetylcholinesterase (at a same acetylcholinesterase concentration); Fig. 17 (right) shows the schematic diagram for the decomposition of acetylcholine catalyzed by acetylcholinesterase. Fig. 18 shows the influence of the solvent system on the amount of the immobilized acetylcholinesterase (n=3). Fig. 19 shows the influence of BSA on the amount of the immobilized acetylcholinesterase. Fig. 20 shows several methods for the detection of the acetylcholinesterase activity. Fig. 21 shows the pH reponses to the decomposition of ATCh catalyzed by acetylcholinesterase. Fig. 22 shows the detection of the enzyme activity by an electrochemical method. Fig. 23 shows the relation curve for the amount of ATCh added V.S. the electricity. Fig. 24 shows a schematic diagram of the pesticide detection by magnetic nanoparticles with acetylcholinesterase loaded on the surface. Fig. 25 shows the curve for the inhibition ratio change of Isoprocarb on acetylcholinesterase at different temperatures with the reaction time. Fig. 26 shows the comparation among three methods for the detection of pesticide content using acetylcholinesterase. Fig. 27 shows the schematic diagram of the structure of the detection device. DETAILD DESCRIPTION OF THE INVENTION Embodiments of the invention will now be described with reference to the accompanying drawings, throughout which like parts are referred to by like references, and in which: Definition The term "pesticide" as used herein means all insecticidal substances, bactericidal substances, harmful animal (or weeds) killing substances, etc., which are applied in the agricultural production to ensure and improve the growth of the plants and crops. In particular, it refers to reagents that are used in agriculture for preventing diseases and insects, regulating plant growth, and weeding. In terms of their main use, pesticides are classified into insecticides, acaricides, rodenticides, molluscacides, bactericides, nematocides, herbicides, plant growth regulators, etc. Based on their origins, pesticides can be classified into mineralderived pesticides (inorganic compounds), biogenetic pesticides (natural organics, antibiotics, and microorganism) and chemically synthesized pesticides, etc. Mineralderived pesticides are pesticides originating h m the inorganic compounds of natural mineral raw materials and petroleum. It includes arsenides, sulfide, compounds containing copper, phosphides and fluorides, and petroleum emulsion, etc. Frequently used varieties include Sulfur suspension, Bordeaux, etc. Biogenetic pesticides refer to pesticides developed using biological resources which include anirnail-originated pesticides, (for example, bisultap, methoprene, insect sex athctant, Trichograrnma, etc.), plant-originated pesticides (for example, pyrethrin, nirnbin, clove oil, ethephon, etc.) and mimrganism-originated pesticides (Jingganmycin, Ba.trytis bassiana, Bacillus thuringiensis, etc.). Chemically synthesized pesticides refer to pesticides artificially synthesized and produced in chemical industry, which has many varieties, are widely used and very efficient. Major chemically synthesized pesticides include organophosphate pesticides, carbamate pesticides, organic nitrogen pesticides, pyrethroid pesticides, organochlorine pesticides and organoflurine pesticides, etc. The organophosphate pesticides specifically include phosphates, monothiophosphates, dithiophosphates, phosphonates, phosphoamides, thiophosphoamides, and pyrophosphates, etc. Preferably, representative organophosphate pesticides are glyphosate, dimethoate and Malaoxon. The carbamate pesticides specifically include N-methylcarbamates and dimethylcarbarnates, etc. Preferably, representative carbamate pesticides are Carbaryl and Isoprocarb. Nanoparticles The term "nanoparticles" as used herein mean particles with particle size between 1 nm to 100 nm, which are also referrd to as superfine particles. Nanoparticles lie beween the microscopic system and the macroscopic system and are a collective consisting of limited number of atoms or molecules. Therefore, nanoparticles have physical and chemical properties such as large surface area, large surface curvature, and show the volume effect, surface effect, quantum size effect and microscopic quantum tunnelling effect. Magnetic Nanoparticles The term "magnetic nanoparticles" as used herein refer to nanoparticles having magnetism, which are also referred to as magnetic nanomaterials. The characteristics of magnetic nanomaterials are different b m conventional magnetic materials, because their characteristic physical length associated with magnetism is right in the order of nanometers, for example: single magnetic domain size, critical size of superpararnagnetism, exchange length, and electronic mean fmz path are in the order of 1-100 nm, when the size of a magnetic substance is comparable with these characteristic physical length, abnormal magnetic properties will appear. For example, when the particle size of magnetic nanoparticles is smaller than the critical size of superparamagnetism, the particles is present in a superparamagnetism state without any coercitive force and remanent magnetism. It is well known that, for bulky magnetic materials (for example, Fe, Co, Ni), multiple domains are normally formed in the body to decrease the demagnetizing energy of the system. A nanoparticle shows a high coercive force when having the critical size of a single domain. Small size effect and surface effect lead to a magnetic nanoparticle with lower Curie temperature. In addition, the magnetic nanoparticle has a saturation magnetization (Ms) lower than that of conventional materials, has a specific saturation magnetization that decreases as size thereof decreases. A magnetic transition may even occur to the magnetic materials when particle sizes thereof are reduced to an order of nanometer. The magnetic nanoparticles of the invention can be prepared by a conventional method known in the art, comprising, but not limited to, a wet chemical method, a chemical vapor method and a physical process. The wet chemical method comprises a precipitation method, a sol-gel method, microemulsion and inverse microemulsion method, a hydrothermal method and a polyol reduction method, etc.; the chemical vapor method comprise chemical vapor deposition , chemical vapor condensation and plasma evaporation method, etc.; the physical process comprises evaporation condensation and a magnetron sputtering method, etc. See, for example, Xiaoyan Zhang, et al., Journal of Magnetic Materials and Devices, Volume 35, Issue 6, December 2004, pages 14-17. The magnetic nanoparticles of the invention are magnetic nanoparticles of alloy, ferrite and I or intermetallic compound. Preferably, the magnetic nanoparticles are magnetic nanoparticles containing iron, cobalt, nickel and I or alloy thereof, preferably Fe304 nanoparticles. In a preferable embodiment, the surface of the magnetic nanoparticles comprises a polar group, preferably citrate or hydroxy. Cholinesterase The tenn "cholinesterases" as used herein refers to the enzymes that hydrolyze various choline esters to produce choline and carboxylic acid. Cholinesterases can be divided into true cholinesterases and pseudocholinesterases. Ture cholinesteras, also referred to as acetylcholinesterases (acetylcholinesterase, EC3.1.1.7), are enzymes that catalyze the hydrolysis of acetylcholine to choline and acetic acid, and their catalytic activity can be inhibited by organophosphates or carbamate pesticides. Ture cholinesterases mainly originate from synaptic cleft of cholinergic nerve teminals in animals, and especially, much of them gather in the postsynaptic membrane folds of the motor end plate; and also exist in cholinergic neurons and red cells. Preferably, the acetylcholinesterase originates fiom electric eels or electric skates. This enzyme has the strongest effect on acetylcholinewith the highest specificity. Pseudocholinesterases comprise propionylcholinesterases and butyrylcholinesterases etc., which are widely present in glial cells, blood plasma, livers, kidneys, and intestines of animals. Pseudocholinesterase has low specificity for acetylcholine, and can hydrolyze other choline esters, such as succinylcholine. Immobilization The term "Immobilization", or "immobilizing", or "immobilize" as used herein, also reffered to as "immobilized", refers to treating an enzyme by a physical or chemical process into a form that is not easily run off h m a carrier. According to the invention, the immobilization of a cholinesterase (especially acetylcholinesterase) onto magnetic nanoparticles can be carried out by any conventional method known in the art, which comprises, but is not limited to, an adsorption method, a chemical crosslink method, a covalent bonding method, a physical embedding method, an electrochemical polymerization method and the molecular self-assembly, and so on. See, for example, Yansheng GAO, et al., Shangdong Chemical Engineering, 2008, Volume 37, No. 4, pages 21 -22 and 30. Collection The term "collection", or "collecting" as used herein means collection to a high concentration with a factor above 1.1 times. In particular, "collecting magnetic nanoparticles" according to of the invention means collecting at least 50%, preferably at least 60%, more preferably at least 70%, more preferably at least 80%, even more preferably at least 90%, e.g., at least 95%, 96%, 97%, 98%, 99%, more preferably at least 99.5%, most preferably 100% of all the added nanaparticles. Enzymatic Activity Detection Method According to the invention, the detection of enzymatic activity can be performed by using a spectrometry, a pH value measurement and an electrochemical method. The term "spectrometry" as used herein, also referred to as "spectrophotometry", is performed as follows: a cholinesterase is inhibited by a pesticide and thus deactivated after contacting with the pesticide, after the addition of DTNB+ATCh, acetylthiocholine is hydrolyzed by the residual cholinesterase to thiocholine and acetic acid, and then DTNB reacts with thiocholine to give TNB, which has the maximal absorption at 412 nrn as detected with a spectrometer. In spectrometry, the higher the speed of the absorbance change at 412 nrn, the faster the increase of the TNB concentration, i.e., the fast the increase of the thiocholine concentration, i.e., the higher the enzymatic activity, and the lower the pesticide content; while the lower the speed of the absorpency change, the slower theincrease of the TNB concentration, i.e., the slower the increase of the thiocholine concentration, i.e., the lower the enzymatic activity, the higher the pesticide content. As a result, the presence or the content of pesticide in the sample to be detected is determined by the absorbance change. Detection by spectrometry is fast with high sensitivity, which is a preferable detection method. The term "pH value measurement mehod" as used herein is performed as follows: ACh (ATCh) is hydrolyzed by acetylcholinesterase to produce acetic acid and choline (thiocholine). As a result, the activity of acetylcholinesterase can be detected by detecting the change in pH value of the solution due to the acetic acid produced in the reaction. For this reason, the applicant analyzed the pH reponses resulted fiom the decomposition of ATCh catalyzed by acetylcholinesterase, as shown Fig. 21. As the reaction proceeds, the pH value of the solution will gradually decrease. However, as the activity of acetylcholinesterase is closely related to the pH value of the solution (as shown in Fig. 15), the reduction of pH value will inhibit the catalytic activity of acetylcholinesterase, thus it will take a longer time for the pH value of the solution to reach a equilibrium (close to 5 hours ). Therefore, detection of the pH change can not lead to fast detection. The term "electrochemical method" as used herein is performed as follows: acetylcholine (acetylthiocholine) is hydrolyzed by acetylcholinesterase to produce acetic acid and choline (thiocholine), thus the activity of acetylcholinesterase can be examined through detecting the oxidation current (inverse current) of choline (thiocholine) by applying a certain voltage. As shown in figures, without the addition of the acetylcholinesterase, there is no obvious oxidation peak appearing in the cyclic voltarnpere curve; with the addition of the acetylcholinesterase, a oxidation peak for thiocholine appears around 0.7 V. Considering that the -OH hctional groups on choline (thiocholine) have a higher oxidation potential, they will interfere with the detection, and thereby acetylthiocholine (ATCh) is usually used as a substrate, ATCh is catalytically hydrolyzed by acetylcholinesterase to form a thiocholine (TCh) with a -SH hctional group and having a oxidation potential of 0.7 V when the electrochemical method is used for detection in the prior art. As shown in Fig. 22, the TCh content in solution can be calculated according to the magnitude of the oxidation current at 0.7 V, then the activity of acetylcholinesterase can be otained. Further, the content of the pesticide can be deduced based on the inhibition rate of the acetylcholinesterase. As shown in Fig. 23, with the amount of the added ATCh increasing, hydrolysis leads to more TCh, causing the increase of the oxdidation current at 0.7 V. In contrast to the detection of the pH value reponse, the electrochemical method can achieve a faster response. As shown in Fig. 23, the response time is about 40 seconds. However, in contrast to the spectrometry, the electrochemical method has poor sensitivity, and consumes a larger amount of acetylcholinesterase and ATCh, which increases the detection cost. Moreover, it should be noted that, the spectrometry of the invention has advantages over the detection methods used in the prior art (comprising a method that directly detects the absorbance of an enzyme solution, and a detection method using electrodes loaded with magnetic nanoparticles and acetylcholinesterase), which is manifested in at least the following aspects: 1. Comparison with the method that directly detects the absorbance of an enzyme solution The direct detection of the absorbance of the enzyme solution is a method in which a solution of free acetylcholinesterases (assuming that the concentration of the enzyme solution is A, the addition volume is VAChEis) directly mixed with the solution to be detected (containing a pesticide and having a volume of Vpesticidet,o) conduct an enzyme inhibition reaction. After a thorough reaction, several volumes of the above solution (Vadd,w) ere added into a volume of DTNB solution (VDm). ATCh (VATChw) as finally added to conduct the spectromic detection. As the concentration of acetylcholinesterase was gradually diluted during the process, the signals to be detected were weakened. The concentration of the acetylcholinesterase at the spectrum detection can be obtained by a calculation according to the following formula: By using the same experimental parameters as above and using the method according to the invention wherein the acetylcholinesterases are enriched using magnetic nanoparticles after the enzyme inhibition reaction and all transferred into the DTNB solution, the concentration of acetylcholinesterase in the spectrometric detection is AS 0 < Vadd. I VAChE+VpeCd,l 8) or too low (for example, pH<6), the immobilization of acetylcholinesterase on the magnetic nanoparticles is negatively influenced. Example 5: Influence of the polymer assisted immobilization on the amount of the immobilized enzyme The present example studied the influence of the polymer assisted immobilization on the amount of the immobilized enzyme. Fig. 16 shows the addition of the polymer V.S. the amount of the immobilized acetylcholinesterase (n=3) during immobilizaion. As shown in Fig. 16, five polymers that are conventionally used in immobilization were chosen (namely, glutaric dialdehyde, gelatin, chitosan, polyethylene glycol and PDDA (poly(diallyldimethylammonium chloride)). According to the prior literatures, acetylcholinesterase is generally mixed with the polymer solution and then coated dropwise onto the surface of the electrodes, and dried, which is then used in detecting the enzyrne activity by an electrochemical method. However, the amount of the actually immobilized enzyme is rarely reported in the literatures. Moreover, since those polymers have a certain solubility in water, a part of the enzyme immobilized on the electrode surface may be re-dissolved during the electrochemical measurement, which will negatively influence the accuracy of the measurement. As reported in the literature, 1 wt.% PDDA, 1 wt.% polyethylene glycol, 1 wt.% chitosan, 1 wt.% gelatin, and 5 wt.% glutaric dialdehyde were mixed, respectively, with 400 pL of 9.225 gL aqueous magnetic nanoparticle solution, and then isolated by centrifugation after an ultrasound for 20 mins to obtain magnetic nanoparticles with their surface being modified with polymer. 1 ml of 1UImL acetylcholinesterase was then added, and an enzyme immobilization was kept at a temperature of 243°C for 20 hours. The free acetylcholinesterase that was not immobilized onto the surface of the magnetic nanoparticles was then removed by centrifugation. Another 4 ml of PBS solution (pH=7.4) was added. After washing and centrifugation, the final acetylcholinesterase immobilized magnetic nanoparticles were stored in 2 ml of deionized water at a store temperature of 2-8°C. Fig. 16 gives the amount of the immobilized acetylcholinesterase assisted by the above five polymers. It can be seen that the amount of the immobilized acetylcholinesterase remarkably decreased after the surface was modified by the polymers, with the highest irnrnbolization amount of only 66.8 Ulg (PDDA), far lower than the immobilization amount without modification with the polymers. In addition, we have found that as PDDA has a quaternary ammonium salt structure similar to the acetylcholinesterase substrate, i.e. acetylcholine (ACh) (interacting with the catalytic active center of acetylcholinesterase), the introduction of PDDA may cause the reduction of the enzymatic activity, as shown in Fig. 17. Example 6: Influence of the solvent system on the amount of the immbolized enzyme The present example studied the influence of the solvent system on the amount of the irnrnbolized enzyme. In this study, it is found that the categories of the solvents used for the magnetic nanoparticle and acetylcholinesterase may also influence the amount of the immbolized enzyme. Since the solution of the magnetic nanoparticles are actually of colloid, the introduction of electrolytes (such as PBS) may result in the coagulation of the magnetic nanoparticles. The coagulation will reduce the contacting area between the magnetic nanoparticles and acetylcholinesterase, and thereby reduce the amount of the enzyme immbolized on the surface of magnetic nanoparticles. Here, we have investigated four kinds of solvent systems: 1. dissolving the magnetic nanoparticles in deionized water - dissolving acetylcholinesterase in deionized water; 2. dissolving the magneticnanoparticles in deionized water - dissolving acetylcholinesterase in phosphate buffer solution; 3. dissolving the magnetic nanoparticles in phosphate buffer solution - dissolving acetylcholinesterase in deionized water; 4. dissolving the magnetic nanoparticles in phosphate buffer solution - dissolving acetylcholinesterase in phosphate buffer solution. All the above four acetylcholinesterase solutions contain l a BSA (bovine serum albumin) as a stabilizer for acetylcholinesterase. Fig. 18 shows the influence of the solvent system on the amount of the immobilized acetylcholinesterase (n=3). As shown in Fig. 18, the phosphate buffer solution of acetylcholinesterase contributes to the immboliztion of acetylcholinesterase onto the surface of magnetic nanoparticles (Samples 2 and 4), while the immobilization amount of sample 4 was only around 50% of that of sample 2 due to the coagulation of magnetic nanoparticles caused by PBS. The present example further investigates the influence of the addition of BSA on the amount of the immobilized acetylcholinesterase, with the exrnperirnental results shown in Fig. 19. In the case that the deionized water @I water) solution of acetylcholinesterase used to immobilize the enzyme is fiee of BSA, acetylcholinesterase fails to be immobilized onto the surface of the magnetic nanoparticles whether the magnetic nanoparticles are dissolved in deionized water or PBS, which indicates that the introduction of BSA not only functions as stabilizing the acetylcholinesterase, but also promotes the immobliztion of the acetylcholinesterase onto magnetic nanoparticles. Example 7: Stability of immobilized acetylcholinesterase and DTNB The stability of immobilized AChE was analyzed by measuring the activity of AChE every day. The stability of immobilized AChE stored at room temperature is comparable or even higher than h e free AChE stored at 4'C, as shown in Fig. 9. Compared to the stability of free AChE stored at room temperature in 15 days, MNPs can enhance the stability of immobilized AChE. The Ellman's reagent, DTNB, also undergoes a photolysis when exposed to sun light, as shown in Fig. 10. But fortunately, the decomposition of DTNB was not found if it was stored at a dark place, even if at a high temperature (around 40°C outdoors in the noon). The good stability of immobilized AChE and DTNB provide us an opportunity to design a pesticide sensor with a long lifespan. Example 8: Measurement of the UV-vis spectrum for acetylcholinesterase In a general measurement, a solution of 4 ml PBS, 1 ml DTNB (1 mM) and 125 pl acetylcholinesterase-immobilized magnetic nanoparticles solution were mixed in a 25 ml plastic bottle. 3 pl ATCh was then added into the bottle and shaked to be homogeneous, and transferred to a cuvette. After the air bubbles were removed by an ultrasound treatment, the reaction solution was immediately (timing at the beginning of adding ATCh, the duration less than 50 seconds) transferred into the spectrometer. Example 9: Inhibition of pesticide on acetylcholinesterase The present example provides the inihibition curve of pesticide to acetylcholinesterase, which illustrates the experimental procedure to obtain the curve and the normalized activity % at different concentrations of the immobilized enzyme. 1. Plotting the inhibition curve of pesticide V.S. acetylcholinesterase 0.25 ml of 1.93 g/L acetylcholinesterase immobilized magnetic nanoparticles solution (with the immobilization amount of 283 Ulg) was added to 1 ml aqueous Isoprocarb (methylethyl phenol methylcarbarnate, a k i d of carbarnate) solution having various concentrations, with the enzyme inhibition time of 40 mins (the experimental parameters were not optimized at the beginning, thereby the enzyme inhibition time was suitably extended to ensure the reaction equilibrium; the late phase of enzyme inhibition can be controlled within 10 min, as shown in Fig. 25). The acetylcholinesterase enzyme in the pesticide solution was then collected by a magnetic field using a quartz ampoule with micro magnet inside, as shown in Fig. 24. Within 60 seconds, the acetylcholinesterase enzyme in the pesticide solution (signal source) could be substantially collected, as the colour change of the solution fiom khaki to colourless shows (low concentration of aqueous magnetic nanoparticles solution appears khaki). After the magnet was removed fiom the quartz ampoule, acetylcholinesterase immobilized magnetic nanoparticles remained on the surface of the quartz ampoule. The magnetic nanoparticles on the surface can be easily re-dispersed into a solution as long as the quartz ampoule is inserted into a DTNB solution due to the water solubility of the magnetic nanoparticles, so as to facilitate the detection of the activity of the acetylcholinesterase immobilized on the surface. 2. The normalized activity of the immobilized enzyme at various concentrations can be obtained as follows: In order to clearly and directly reflect the inhibition effect of the pesticide to the acetylcholinesterase activity, the acetylcholinesterase activity detected without pesticide was defined as 100%. The other points were normalized based on the acetylcholinesterase activity at the pesticide concentration of 0, thereby providing Fig. 12. Example 10: Optimization of the parameters in inhibition experiments In order to further shorten the time of the enzyme inhibition reaction and thereby make a fast detection, the experimental parameters for the inhibition of pesticide on enzyme were optimized in the example. It has been found in the research that, at a certain concentration of pesticide, the reaction speed of inhibiting acetylcholinesterase is closely related to the temperature. If the temperature of the enzyme inhibition reaction is suitably increased, the enzyme inhibition reaction can reach an equilibirium within 10 minutes (at 40°C, as shown in Fig. 25). It only takes about 3 minutes to reach 50% of the inhibition reaction at equilibirium at 40°C for 200 pg/L of Isoprocarb. By collecting acetylcholine esters using the magnetic nanoparticles, the signal strength of the detection is remarkably enhanced, which means that the enzyme inhibition reaction can be detected before reaching an equilibirum, thereby eventually realizing a fast detection of the pesticide residue level. Furthermore, it can also be seen fiom the research that if the temperature of the enzyme inhibition reaction exceeds 60°C, the configuration of acetylcholinesterase will change, resulting in a complete loss of the catalytic activity to the subtrate. Therefore, the temperature of the enzyme 31 inhibition reaction should be adjusted to 0-60°C. In Fig. 25, the concentration of Isoprocarb was 200 pgiL, and the concentration of acetylcholinesterase was 0.2 Ulml. Example 11: Pesticide detection using acetylcholinesterase immobilized magnetic nanoparticles The present example provides an embodiment of pesticide detection using acetylcholinesterase immobilized magnetic nanoparticles. A 0.25 ml solution of acetylcholinesterase immobilized magnetic nanoparticles was added to a solution of a sample containing pesticide, and incubated for 40 mins. The acetylcholinesterase immobilized magnetic nanoparticles were then collected by a small magnet covered with a glass tube. Within 100 seconds, almost all acetylcholinesterase immobilized magnetic nanoparticles can be collected. After the removal of the magnet from the glass tube, the acetylcholinesterase immobilized magnetic nanoparticles could be re-dispersed into a solution containing 4 ml PBS @H=7.4) and 1 ml DTNB. After the addition of ATCh, the speed of the absorbance change was detected around 412 nm with a UV-vis spectrometer. Example 12: Influence of the pH value at detection on the stability of acetylcholinesterase The present example studied the influence of the pH value at detection on the stability of acetylcholinesterase. Fig. 15 shows the influence of the pH value of acetylcholinesterase (in a free state, without being immobilized on the surface of magnetic nanoparticles) on activity at detection. Fig. 15(a) shows the absorbance change at 412 nm at different pH values, wherein the slope of UV-vis spectrometry spectrum curve changing with the pH value reflects the enzymatic activity changing with the pH value; Fig. 15(b) shows the result after a normalization based on the highest point (at pH=9) of the enzymatic activity varying from pH 2 to pH 10, wherein it is found during the detection that DTNB will spontaneously and quickly decompose in a strong basic circumstance at a pH of 1 1, even no enzyme is added. Therefore, the total apparent activity is enhanced. However, if the influence imposed by the high pH value is excluded, the actual activity of the enzyme is slightly decreased as compared with that at a pH = 10. 32 Fig. 8 (b) shows the influence of the pH value of the detection system added with 125 p1 of acetylcholinesterase (1 Ulml) solution on the activity of acetylcholinesterase. The results show that the slope (K) of the UV-vis spectrum increases as the pH value increases, as shown in Fig. 8 (b). A larger K value may increase the signal-noise ratio, and also shorten the detection time. However, the pesticide tends to hydrolyze if the pH value exceeds pH 8. In order to avoid the hydrolysis of the pesticide, we chose a pH value of 8 in the present invention. Example 13: the influence of the volume of acetylcholinesterase added at detection on the response time The present example studied the influence of the volume of acetylcholinesterase added at detection on the response time. In practice, we need the pesticide sensor not only with high sensitivity and long stability, but also with a fast response. To achieve a fast detection, the detection system based on h e AChE has been optimized. The first parameter optimized was the volume of AChE (1 Ulml) added to PBS (pH=7.4) in detection, as shown in Fig. 8a. When the volume of AChE was only 25 &, the hydrolyzation under the catalysis of AChE started at 7 min after the ATCh adding. With the increase of ACE added, the initial reaction time decreased and when the volume of AChE was up to 125 &, the hydrolyzation started immediately after the ATCh adding. Further increasing the amount of AChE did not shorten the reaction time, whereas the more AChE added, the more ATCh consumed to make sure that the hydrolysis rate of ATCh is determined by the AChE concentration. Considering the cost issue, 125 p1 of AChE (1 Ulml) solution or the equivalent, 125 mu ACE, was employed in each detection. Example 14: Experimental results of pesticide detection The present example provides experimental results of pesticide detection by acetylcholinesterase-immobilized magnetic nanoparticles. Five pesticides have been examined and three of them (Carbaryl, Isoprocarb and Malaxon) can inhibit the activity of AChE, as shown in Fig. 11. Herein, we further analyzed the inhibition effect vs. the concentration of carbaryl. Results show that the inhibition of AChE-immobilized MNPs has a good linearity with the concentration of carbaryl in the range from 50 (or even lower) to 200 pgL, as shown in Fig. 12. According to the U.S. Environmental Protection Agency, the maximum residue ' limit of carbaryl is 100 pg/L, which can be easily detected. Example 15: A portable pen-like detector for pesticide detection The present example provides a portable pen-like detector for pesticide detection, comprising the following elements: a battery (1) for providing electricity to chips (2) and (1 1); a chip (2) which is connected to a light emitting diode (8) and controls it for providing light source required by the spectrum detection; a button (3), connected to the container (5) for controlling the addition of DTN3; a button (4), connected to the container (6) for controlling the addition of ATCh; a container (9, connected to the cuvette (9) under a control of button (3) for the addition and storage of DTNB; a container (6), connected the cuvette (9) under a control of button (4) for the addition and storage of ATCh; a liquid crystal screen (7), connected to the data process chip (1 1) for displaying the detection results; a light emitting diode (8) as a luminescent element which is controlled by the chip (2) for providing a wavelength (390-430 nm, with 412 nm optimal) required by the spectrum detection; a cuvette (9) for inserting into the magnet device(l4) and adding DTNB and ATCh, so as to perform the spectrum detection in the cuvette (9); a photoelectric conversion device (10) connected to the chip (1 1) for converting the collected optical signal to electrical signal; a chip (1 1) connected to the photoelectric conversion device (1 0) and the liquid crystal screen (7) for processing and transferring data, a button (12) connected to the container (13) for controlling the release of the acetylcholinesterase-immoblized magnetic nanoparticles solution; a container (13), which is controlled by the button (12) for adding and storing the acetylcholinesterase immobilized magnetic nanoparticles solution; a magnet device (14), comprising a casing (14a) made of glass or polytetrafluoroethylene and an inside magnet (14b), wherein the magnet device(l4) can be taken out of the detector for collecting and transfening the magnetic nanoparticles and can be inserted into the cuvette (9) for the spectrum detection. Example 16: Use of the detector in Example 15 for detecting pesticide The present example illustrates the operation procedure of pesticide detection using the detector in Example 15. Button 12 was pressed, and acetylcholinesterase imrnbolized magnetic nanoparticles solution was dropped onto the surface of the vegetable to be detected with an area of approximately 0.5 cm2. 3 minutes later, magnet 14 (jacketed with glass or polytetrafluoroethylene) was used to collect the magnetic nanoparticles in the liquid drops for a collection time of approximately 30 s. Then the magnet 14 was inserted into the microcuvette 9 followed by removing the permanent magnet inside 14, pressing the button 3, and washing the surface. DTNB and acetylcholinesterase-magnetic nanoparticles were transferred to the cuvette 9. After connecting to the electricity, when the readings on the LCD became stable, pressing the button 4 to add ATCh into the cuvette. The speed of the color change in the cuvette was processed by the chip 1 1, and one minute later, the concentration @dm2) of the pesticide on the vegetable surface was directly displayed on the LCD 7. The invention provides a novel pesticide detection method and a new detection device involving acetylcholinesterase-immobilized nanoparticles. Those soluble magnetic nanoparticles work as an acetylcholinesterase carrier in detection, which can be easily collected by a magnet or removed by water. The high stability and good response of immobilized AChE on MNPs establish a solid basis for the design of a pesticide sensor with high sensitive and fast response. Although the invention are illustrated and described with the reference to the specific examples according to the invention, an ordinary skilled in the art would understand that any modification of various forms can be made to the invention without departing the spirit and scope of the invention and the attached claims. CLAIMS 1. A method for pesticide detection, comprising the steps of: (a) providing a sample to be detected; (b) preparing magnetic nanoparticles, and contacting cholinesterase with the magnetic nanoparticles to obtain cholinesterase-immbolilized magnetic nanoparticles; (c) adding the cholinesterase-immbolilized magnetic nanoparticles obtained in step (b) to the sample to be detected of step (a), and incubating; (d) collecting the magnetic nanoparticles by a magnetic field; and (e) removing the magnetic field, dispersing the collected magnetic nanoparticles in an enzymatic activity detection system, dectecting the enzymatic activity of cholinesterase, and determining the presence or the content of the pesticide based on the enzymatic activity. 2. The method according to claim 1, wherein the pesticide is a chemically synthesized pesticide. 3. The method according to claim 1, wherein the pesticide is selected from the group consisting of: organophosphates pesticides, carbamates pesticides, organic nitrogens pesticides, pyrethroids pesticides, organochlorines pesticides and organoflurines pesticides. 4. The method according to claim 3, wherein the organophosphates pesticide is selected fi-om the group consisting of: phosphates, monothiophosphates, dithiophosphates, phosphonates, phosphoamides, thiophosphoamides, and pyrophosphates. 5. The method according to claim 3, wherein the organophosphates pesticide is sleeted from: glyphosate, dimethoate and Malaoxon. 6. The method according to claim 3, wherein the carbamates pesticide is selected from the group consisting of: N-methylcarbamates and dimethylcarbamates. 7. The method according to claim 3, wherein the carbamates pesticide is selected from Carbaryl and Isoprocarb. 8. The method according to any of claims 1 to 7, wherein the magnetic nanoparticles is prepared by a method selected from the group consisting of: a wet chemistry method, a chemistry vapor method and a physical method. 36 9. The method according to claim 8, wherein the wet chemistry method is selected from a precipitation method, a sol-gel method, a microemulsion and inverse microemulsion method, a hydrothermal method and a polyol reduction method; the chemistry vapor method is selected from a chemical vapor deposition, a chemical vapor condensation and a plasma evaporation method; and the physical process is selected from an evaporation condensation method and a magnetron sputtering method. 10. The method according to any of claims 1 to 9, wherein the magnetic nanoparticle is seleted from the magnetic nanoparticles of alloy, ferrite and / or intermetallic compound. 11. The method according to any of claims 1 to 10, wherein the magnetic nanoparticle is magnetic nanoparticles containing iron, cobalt, nickel or the alloy thereof, preferably Fe304 nanoparticles. 12. The method according to any of claims 1 to 11, wherein the surface of the magnetic nanoparticle comprises polar group, preferably citrate or hydroxyl group. 13. The method according to any of claims 1 to 12, wherein the cholinesterase is selected from the group consisting of: acetylcholinesterase, propionylcholinesterase and butyrylcholinesterase. 14. The method according to claim 13, wherein the acetylcholinesterase is obtained from a source selected from the group consisting of: microorganism, plants and animals, preferably electric eels or electric skates. 15. The method according to any of claims 1 to 14, fiirther comprising a step of calibrating the magnetic nanoparticles, preferably by a standard curve, prior to step (b). 16. The method according to any of claims 1 to 15, fiirther comprising a step of calibrating concentration of cholinesterase, preferably by a standard curve, prior to step (b). 17. The method according to any of claims 1 to 16, fiirther comprising a step of washing the magnetic nanoparticles, preferably using deionized water, for 1-5 times, more preferably for 4 times, prior to step (b). 18. The method according to any of claims 1 to 17, wherein the immobilization of cholinesterase onto magnetic nanoparticles in step (b) is carried out by a method selected from the group consisting of: adsorption method, chemical crosslinking method, covalent linking method, physical embedding method, elecfrochemical polymerization method and molecular self-assembly method. 37 19. The method according to any of claims 1 to 18, wherein the immobilization of cholinesterase onto magnetic nanoparticles in step (b) is carried out under a condition of pH 6-8, preferably pH 7. 20. The method according to any of claims 1 to 19, wherein during the immobilization of cholinesterase onto magnetic nanoparticles in step (b), cholinesterase and the magnetic nanoparticles are added in a ratio of 20~330:1 U/g, preferably 125~320:1 U/g, more preferably 250-310:1 U/g, more preferably 280:1 U/g. 21. The method according to any of claims 1 to 20, wherein the immobilization of cholinesterase onto magnetic nanoparticles in step (b) is carried out as follows: (A) dispersing the magnetic nanoparticles in deionized water or a phophate buffered solution; (B) dissolving cholinesterase in deionized water or a phophate buffered solution containing bovine serum albimiin (BSA); and (C) contacting the magnetic nanoparticles solution with the cholinesterase solution. 22. The method according to any of claims 1 to 21, wherein the immobilization of cholinesterase onto magnetic nanoparticles in step (b) is carried out as follows: (a) dispersing the magnetic nanoparticles in deionized water; (b) dissolving cholinesterase in a phophate buffered solution containing bovine serum albumin (BSA); and (c) contacting the magnetic nanoparticles solution with the cholinesterase solution. 23. The method according to any of claims 21 to 22, wherein the concentration of the magnetic nanoparticles solution is 0.1-10 g/L, preferably 0.5-5 g/L, more preferably 1-3 g/L, more preferably approximately 2 g/L. 24. The method according to any of claims 21 to 22, wherein the concentration of cholinesterase solution is 0.1-10 U/ml, preferably 0.5-2 U/ml, more preferably 1 U/ml. 25. The method according to any of claims 21 to 22, wherein the concentration of bovine serum albumin in phosphate buffered solution is 0.1-5 g/L, preferably 0.5-2 g/L, more preferably 0.8-1.2 g/L, more preferably 1 g/L, 26. The method according to any of claims 1 to 25, wherein 0.05-1 ml, preferably 0.1-0.5 ml, more preferably 0.25 ml of cholinesterase-immbolilized magnetic nanoparticles solution is added into the sample to be detected in step (c). 38 27. The method according to any of claims 1 to 26, wherein, in step (c), the sample to be detected and the cholinesterase-immbolilized magnetic nanoparticles are incubated at a temperature of 0-60°C, preferably 20-50*'C, more preferably 40°C for 2 minutes - 2 hours, preferably 20 minutes -1 hour, more preferably 40 minutes. 28. The method according to any of claims 1 to 27, wherein the magnetic nanoparticles are collected by a magnet coated with an outer layer in step (d). 29. The method according to claim 28, wherein the diameter of the magnet is approximately 2-10 mm, preferably 3-8 mm, more preferably 4 mm. 30. The method according to any of claims 1 to 29, wherein the step (d) is performed for 1-10 minutes, preferably 1-5 minutes, more preferably 100 seconds. 31. The method according to any of claims 1 to 30, wherein at least 60%, preferably at least 80%, more preferably at least 90%, more preferably substantially 100% of the magnetic nanoparticles are coUeced in step (d). 32. The method according to any of claims 1 to 31, wherein the enzymatic activity detection system in step (e) has a pH value of 2-11, preferably 6-10, more preferably 7-9. 33. The method according to any of claims 1 to 32, wherein the detection of the enzymatic activity of cholinesterase in step (e) is performed by a pH value measurement method, an electrochemistry method or a spectrophotometry method, preferably by the spectrophotometry method. 34. The method according to claim 33, wherein, in step (e), 5,5'-dithiobis-(2-nitrobenzoic acid) (DTNB) and thiocholine (TCh) are added for detecting the enzymatic activity of cholinesterase by the spectrophotometry method. 35. The method according to claim 34, wherein the spectrophotometry method is used to determine the speed of absorbency change; the higher speed of absorbency change indicates a higher enzymatic activity, whereas the lower speed of absorbency change indicates a lower enzymatic activity. 36. The method according to any of claims 1 to 35, wherein the higher enzymatic activity determined in step (e) indicates a smaller amount of pesticide contained in the sample to be detected, whereas the lower enzymatic activity determined in step (e) indicates a larger amoimt of pesticide contained in the sample to be detected. 39 37. A portable detector for pesticide detection, comprising: a power supply system, a storage system, a control system, and a determination and analysis system. 38. The detector according to claim 37, wherein the power supply system is a battery (1), which is used for supply electricity to other systems of the detector. 39. The detector according to any of claims 37-38, wherein the storage system comprises a DTNB container (5), a thiocholine container (6), a magnetic nanoparticle solution container (13), and a magnet device (14) provided with a magnet for collecting and transferring the magnetic nanoparticles. 40. The detector according to claim 39, wherein the magnet device (14) comprises a casing (14a) and an inside magnet (14b), wherein the inside magnet (14b) is removable from the casing (14a). 41. The detector according to claim 40, wherein the casing (14a) is made of glass or polytetrafluoroethylene. 42. The detector according to any of claims 37-41, wherein the control system comprises a button (3) connected to the DTNB container (5) for controlling the addition of DTNB, a button (4) connected to the thiocholine container (6) for controlling the addition of cholinesterase, and a button (12) for controlling the release of the cholinesterase-immbolilized magnetic nanoparticles solution. 43. The detector according to any of claims 37-42, wherein the determination and analysis system comprises: a light emitting diode (8) that provides the wavelength required by the spectrum detection; a chip (2) cormected to the light emitting diode (8) for controlling the light emitting diode; a cuvette (9) that conntected to the DTNB container (5) through the button (3), and connected to thiocholine container (6) through the button (4), and into which the magnet can be inserted; a photoelectric conversion device (10) for converting the collected optical signal to electrical signal; a liquid crystal screen (7) for displaying the detected results; and a data processing chip (11) connected to the photoelectric conversion device (10) and the liquid crystal screen (7), repectively. 44. The detector according to any of claims 37-43, wherein the detector is pen-like. 45. A method of pesticide detection using a detector as defined in any of claims 37 to 44, 40 comprising the steps of: (a) pressing button (12), so that a solution of cholinesterase-immbolilized magnetic nanoparticles is contacted with a sample to be detected; (b) collecting the magnetic nanoparticles in the sample by a magnet device (14), inserting the magnet device (14) into cuvette(9), and then removing the inside magnet (14b) from the magnet device(14); (c) pressing button (3), adding DTNB and the cholinesterase-immbolilized magnetic nanoparticles on the casing (14a) of the magnet device(14) into the the cuvette (9); (d) connecting to the electric supply, and pressing button (4) to add acetylthiocholine into the cuvette after the reading on the liquid crystal display screen becomes stable; and (e) determining the concentration of the pesticide in the sample according to the color change speed of the cuvette by a determination and analysis system, and displaying on the iquid crystal display screen (7). 46. The method according to claim 45, wherein step (a) is carried out for 1-5 minutes, preferably 3 minutes. 47. The method according to claim 45, wherein the collecting in step (b) is carried out for 10-60 seconds, preferably 30 seconds. 48. A use of a detector according to any of claims 37 to 44 for pesticide detection. Dated this 11/1/2012