Claims:We Claim:
1. Self assembled metal nanowires, wherein thelength of the nanowires is ranging from about 1µm to 2.8µm and thickness ranging from about 100nm to about 200nm; providing a substrate coverage ranging from about 75% to about 80%.
2. Self assembled metal nanowires, as claimed in claim 1,wherein the metal nanowires are selected from a group comprising Au, Ag preferably Au nanoparticles.
3. A method for growing metal nanowires wherein length of nanowires is ranging from about 1µm to 2.8µm and thickness ranging from about 100nm to about 200nm,said method comprising acts of
i. synthesising seeding solution at temperature of about1°C to about 5° ,comprising metal nanoparticles, further comprising acts of
preparing HAuCl3 andd NaBH4(aq.) solution separately;
mixing Tri-sodium citrate solution with HAuCl3 and
adding NaBH4 solution to obtain the seeding solution comprising metal nanoparticles
ii. coating the seeding solution comprising metal nanoparticles, onto the substrate, at temperature ranging from 20°C to 30°C preferably 24°C, perpendicular to the structured substrate to grow the metal nanowires on the structured substrate;
wherein the structured substrate is maintained at an angle of ranging from about 30° to about 45°, preferably 45°; and
iii. evaporating the solution to obtain self assembled metal nanowires
4. A solution comprising metal nanoparticles for preparing self assembled nanowires with a stability ranging from about 100 days to about 150 days preferably 145 days.
5. A sensor fabricated with self assembled metal nanowires of claim 1, wherein the sensor is used for biosensing, electromechanical electrochemical sensing applications and the like.

, Description:Field of invention:
The present invention relates to the field of sensors. More specifically, the invention relates to self assembled nanostructures on substrates. In particular, the invention relates to nanowires grown on structured substrates and also a method for obtaining the self assembled nanowires across a wide surface area to be fabricated for using in various sensing applications.

Background of invention:
Advancement in the field of nanotechnology, has led to a new approach to devise nanomaterials and other devices, for various applications. Realization of the nanotechnology is done through simple efficient methods of organizing the nanomaterials into precise nanostructures. Assembling nanoparticles into nanostructures has opened up techniques of fabricating nanomaterials with enhanced physical and chemical properties and applications.
Conventionally, e-beam lithography and Ion beam lithography are used to achieve fabrication of nanostructures on patterned substrates. However, the methods are expensive and provide limited substrate coverage. Also, the use of fabrication equipment’s for example, electron beam pattern generators, ion beam tools and the like is increasingly costly. Similarly, photolithography technique is used for fabricating micro size features by using highly focused and well controlled UV light and UV-sensitive photoresists. Pre patterned chrome masks are made using laser writing technique.In nanoimprint lithography technique, transference of pattern is achieved through embossing at suitable temperature and pressure. These imprint fabrication methods use laser writing UV lithography or e-Beam lithography as a pre-process for fabricating the mould. Other processes use functionalization of surfaces for the selective adsorption of chemical species on the substrate. These methods are expensive and increase the complexity of process of fabrication of the nanostructures.
Nanostructures have been fabricated either through controlled assembly or through self assembly. For example Cadmium sulphide nanoparticles can be synthesised into rod-like nanostructures through controlled assembly using experimental parameters for example by modifying temperature , pH and the like. The ability to attach nanostructures at precise location and in an exact pattern has been a challenging task for all practical applications. Nanostructures are grown on substrates to obtain a specific pattern. Regarding required positioning, however, it is complicated to achieve the desired result, due to their small size. Self assembly allows fabrication of the nanostructures with less effort and complexity than any other fabrication methods Developing methods which allow self assembly of nanostructures with adequate precision and repeatability can reduce manufacturing costs to a considerable extent .
Patent document number US 20130213265 titled “Self-Assembly of Metallic Nanoparticles into Macroscopic, High-Density, Monolayer Films” discloses a method of forming a monolayer film of nanoparticles. This comprises forming a fluid mixture by combining nanoparticles dispersed in water with a watermiscible organic solvent and a molecular ligand comprising a head group with affinity for the nanoparticle, and introducing the fluid mixture to a substrate in the presence of an air/fluid interface, thereby causing a monolayer film of nanoparticles to form on the substrate. Such monolayers films can include metallic nanoparticles such as gold, and possess substantially uniform spacing over at least a one centimetre length scale
Patent document number US 20090082216 titled “Metallic nanostructures self-assembly, and testing methods” discloses metallic nanostructures self-assembly methods and materials testing. The methods permit for the formation of individual nanostructures and arrays of nanostructures. Example structures include slender wires, rectangular bars, or plate-like structures.
Patent document number US 20100093160 titled “Methods of forming nano-devices using nanostructures having self assembly characteristics” provides methods of forming nano-devices. One of the methods includes forming a nano-scale self-assembly material layer on a substrate formed of at least one layer, forming a mask layer on the self-assembly material layer, performing a surface treatment process on the substrate using the mask layer as a template, and removing the self-assembly material layer.
Patent document number US 20030180472 titled “Method for assembling nano objects” discloses a method for the self assembly of a macroscopic structure with a preformed nano object is provided. The method includes processing a nano object to a desired aspect ratio and chemical functionality and mixing the processed nano object with a solvent to form a suspension. Upon formation of the suspension, a substrate is inserted into the suspension. By either evaporation of the solvent, changing the pH value of the suspension, or changing the temperature of the suspension, the nano objects within the suspension deposit onto the substrate in an orientational order. In addition, a seed crystal may be used in place of the substrate thereby forming single-crystals and free-standing membranes of the nano-objects.
The aforementioned documents provide for methods for the self assembly of nanostructures However, the inventions have certain drawbacks for example limited substrate coverage offered by the self assembled nanostructures. The costs associated with the fabrication of the nano-assemblies is expensive. Agglomeration of the metal nanoparticles during the self assembly of the nanostructures may also be observed. Formation of one dimensional nanostructure in a pre-defined manner is a challenging task.
The present invention overcomes the drawbacks of the earlier inventions by providing for self assembled one dimensional nanostructures with a broad substrate coverage. The invention is designed to provide cost effective method for growing the nanostructures with the ease of fabrication of theself assembly and also avoids agglomeration of the nanoparticles as well.

Summary of invention:
Accordingly, the present invention relates to self assembled metal nanowires, wherein the length of the nanowires is ranging from about 1µm to 28.5 µm and thickness ranging from about 100nm to about 200nmand providing a substrate coverage ranging from about 75% to about 80%. The invention provides for a method for growing the nanowires comprising acts of synthesising a seeding solution at temperature ranging from about 1°C to about 5°comprising metal nanoparticles , coating the seeding solution comprising metal nanoparticles, at temperature of 24°C, onto the substrate maintained at an angle of 45°, and evaporating the solution to obtain self assembled metal nanowires. The invention also provides for a solution comprising metal nanoparticles for preparing self assembled nanowires with a stability of about 145 days.

Brief description of figures:
The features of the present invention can be understood in detail with the aid of appended figures. It is to be noted however, that the appended figures illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope for the invention.
Figure 1 provides a schematic diagram of self-assembly process on DVD grooves. (a) Au seed solution is dropped on cleaned DVD substrate, (b) drop flows down a slanted DVD, (c) self assembled area after drying up the seed solution.
Figure 2(a) and 2(b) shows the inline and cross-line orientations of the drop with respect to the groove. (c) shows slanted substrate at slant angle in an inline orientation.
Figure 3 describes dependence of temperature of seed solution and on self assembly. SEM image is shown from central region of drop cast area when (a) both seed solution synthesis and drop casting is carried out at a temperature of 24C?. (b) when both seed solution which is synthesised and drop casting is carried out at a low temperature of 3?C (c) When seed solution is synthesized at low temperature of (2- 3 ?C) and drop casting is carried out in room temperature of 24C?.
Figure 4 shows schematic representation of drop cast area at 45? slant angle and the corresponding? scanning electron micrographs (SEM) images from respective labelled areas. Good coverage density, and long nanowires seen in region marked c of the figure.
Figure 5 describes dependence of orientation. SEM image is shown from central part of drop casted area on a (a) cross- line channeled substrate at 45? slant angle, (b) in- line channeled substrate at 45?slant angle.
Figure 6 describes dependence of slant angle: SEM images is shown from central part of drop casted area. The samples are grown at the following slant angles (a)0?, (b) 15??(c) 30?and (d) 45?.
Figure 7 shows SEM images from different portions when drop casting done at room temperature with commercially available solution seed solution (already reported CJ murphy’s method) maintaining an angle 45?
Figure 8.Intensity of Raman signal from self assembled nano wire on DVD substrate; Rhodamine 6G loading of varying concentrations are tried. Amplification of Rhodamine's Raman spectra is certainly observed.
Figure 9 provides the Zeta potential measurements for accessing the stability of the freshly prepared seed solution ( figure-9(a)) and the seed solution after 145 days (figure 9(b)).
Detailed description of invention
The foregoing description of the embodiments of the invention has been presented for the purpose of illustration. It is not intended to be exhaustive or to limit the invention to the precise form disclosed as many modifications and variations are possible in light of this disclosure for a person skilled in the art in view of the figures, description and claims. It may further be noted that as used herein and in the appended claims, the singular forms "a", "an", and "the" include plural reference unless the context clearly dictates otherwise. Unless defined otherwise, all technical and scientific terms used herein have the same meanings as commonly understood by person skilled in the art.
The present invention is in relation to self assembled metal nanowires wherein the nanostructures are grown on structured substrate. The invention provides for structured substrate based approach for making metal nanowires; metal nanoparticles of size ranging from about 2nm to 6nm are, nucleated at temperature ranging from about 1°C to 5°Care used for self-assembly in a drop-casting set up. This method can be extended to other similar material systems, by using modified experimental parameters such as number of drop castings, substrate angle and the like. The method shown here uses self assembly of gold(Au) nanoparticles of about 5 nm in size to form long nanowires. The impact of experimental parameters including substrate angle, self assembly temperature, seed solution temperature, orientation of structured substrate, periodicity and number of drop castings on nanowire assembly have been analysed. Nanowires ranging from 1µm to 28µm in length and thickness ranging from about 100nm to about 200nm are obtained using this approach. The parametric studies show that the self-assembly method works through kinetic controls and drying dynamics that are exercised through (i) low temperature solution synthesis, followed by (ii) in-line drop casting on structured substrate,aligned at a particular angle.

The present invention provides for self assembled nanowires on a structured substrates and also the method for synthesis of metal nanoparticle solution and the subsequent drying of the solution on the substrate maintained at an angle of 45?leading to the formation of one dimensional self assembled nanowires.
In one embodiment of the invention. the self assembled metal nanowires are of lengthranging from 1µm to 28µm in length and thickness ranging from about 100nm to about 200nm
In another embodiment of the invention, the self assembled nanowires provides a substrate coverage ranging from about 75% to about 80%.
In another embodiment of the invention, the metal nanowires are selected from a group comprising gold (Au), silver (Ag)preferably gold(Au) nanoparticles.
The present invention is also in relation to a method for growing metal nanowires wherein length of nanowires is ranging from 1µm to 28µm in length and thickness ranging from about 100nm to about 200nm,said method comprising acts of synthesising seeding solution at temperature ranging from about 1°C to about 5°,comprising metal nanoparticles; further comprising acts of preparing HAuCl3 and NaBH4(aq.) solution separately; mixing Tri-sodium citrate solution with HAuCl3 ; followed by adding NaBH4 solution to obtain the seeding solution of Au; coating the seeding solution comprising metal nanoparticles, onto the substrate, at temperature ranging from 20°C to 30°C preferably 24°C, wherein substrate is maintained at an angle of 45°,followed by evaporating the solution to obtain self assembled metal nanowires wherein substrate is maintained at an angle of 45°.
The present invention also provides for a solution comprising metal nanoparticles for preparing self assembled nanowires with a stability ranging from about 100 days to about 150 days preferably 145 days
The present invention also provides for a sensor fabricated with self assembled metal nanowires, wherein the sensor is used for biosensing applications, electromechanical and electrochemical sensing applications and the like.
The method of synthesis of the seeding solution comprising Au metal nanoparticle and the also the process of self assembly of Au nano particles to form a nano wires is explained in the description given below:
Experimental:
A) Synthesis of stable low temperature Au solution by modified C J Murphy’s method:
A solution of 5 mM AuCl3 in 0.1 M HCl is prepared and stored as stock solution at a temperature 1°C to about 4 °C. A 0.2 M NaBH4 (aq.) solution is also prepared and stored as stock solution at about 2°C (ice cold solution). 20 ml of deionized water is added to 1 ml of HAuCl3 drawn from the stock solution. Subsequently 0.5 mM tri-sodium citrate is added to the solution, while maintaining the temperature from about1°C to about 5°. The process is carried out along with magnetic stirring at 700 rpm. 600?l of NaBH4 solution drawn from the stock solution is added to this solution under vigorous magnetic stirring (1500 rpm), to yield a pale red colour dispersion, due to the formation of Au seeds. The Au solution prepared using this method, when stored at 3-4 °C, remains stable for over 4 months. This seed solution is further used for the self-assembly process.
Self-assembly process of Au nano particles to form a nano wire: DVD disks made of polycarbonate; are split into two-halves while ensuring that there is no damage to the circular grating structure of pits and grooves. The DVD halves are carefully separated and used as the substrate. The substrate contains a dye-containing recording layer,. The substrate is immersed in methanol solution and sonicated for about 6 minutes and then sonicated in isopropyl alcohol (IPA) for about 4 minutes in order to remove the recording layer on the substrate.. The self-assembly is carried out on the exposed grating structure; the polycarbonate substrates used for this are 1cm² in area. Figure-1 is a schematic representation of the processes involved in the self-assembly process. To assemble the nanowires, it is important to place the substrate in an inclined position, with respect to the horizontal plane. 20 µl of the seed solution prepared using method described above is dropped on the substrate. Figure-1(b) represents the solution flow and evaporation, which takes about 4 hours. Examination of the dried sample reveal that Au nanowire assembly happen along the grooves (Figure-1(c)). Figure-2 represents the SEM image of the self assembled area from different portions of drop cast area. Figure 2 shows how the orientation and substrateangle affect the formation of nano wire. Figure-2(a), 2(b) show the structures grown when the orientation is cross-line, and in-line respectively. Figure 2(c) shows an in-line channel substrate kept at anangle of45°C; this angle is well suited for the formation of nanowires along the grooves of the substrate.
Various experimental parameters considered for carrying out the growth of nanowires on the substrate is explained below:
Temperature:
In order to form a stable dispersion of the particles, the solution is required to be synthesised at a low temperature. The subsequent assembly of the rods requires a certain energy and if the experiment is performed at lower temperatures, assembling will either (i) require longer durations or (ii) depending on the temperature, the state might be such that the configuration of the particles is topologically stuck with no coherent adhering of the particles. Therefore, the assembly, when performed at a specific temperature provides enough energy to overcome the energetic barriers to assemble, leading to the formation of well-defined nanorods.
(a). Experimental results when self assembly process is carried out ata temperature of 22-24??C at a 45? angle) by using a normal seed solution which is synthesized and preserved at temperature of 24 ??C..
Figure 3a shows the experiment results after the drop casting of 25µl of seed solution has been done under this particular condition. The experiment including drop casting, evaporation and thus self assembly is carried out at temperature ranging from 22??C-24??Cat a substrate angle of 45?? . The seed solution used in this experiment is synthesized by modified CJ Murphy's method. After 10-12 hour of drop casting, the entire drop evaporates; SEMs from different regions of the drop cast areas show that the central region is favourable for nanowire formation. Nanowire formation is not observed from any other region of the drop cast area in this particular condition.

(b) Experimental results when self assembly process is carried out at a low temperature of 2-3?C and an angle of 45? by using seed solution synthesized using the adapted andmodified CJ Murphy's method; the solution used is preserved at low temperature 2-3?Ctemperature..
Figure 3b shows SEM images from drop cast areas of the structured substrate, after the self assembly is carried out at a temperature of 2?C -3?C. and the substrate is aligned at an angle of 45?. The seed solution used in this experiment is preserved at a temperature of 2?C -3 ?C. Figure 4 shows the SEM images from central drop cast area. Although hollow branched-like structures along the grooves is observed, solid nanowire formation is not visible from any region of the drop cast area at this particular condition also.

(c). Experimental results when self assembly process (i.e growth) is carried out at temperature of (24?C with substrate angle of 45? by using seed solution synthesized using adapted and modified CJ Murphy's method ; the preservation of the Au solutionis done at low temperatures of 2-3 C)-
The SEM images in Figure 3c show that the solid nanowire formation occurs primarily at the center. Only this condition guarantees a solid self assembled nanowire formation. At 45? angle, the area of self assembled nano wire formation is about 75-80% of total drop cast area. Figure 4 shows the SEM images of nano wire formation from different regions of drop casted area. Figure 4 (b, c) are SEM images from the central region of the drop casted area, having a solid nano wire formation along the structured substrate.
B) Substrate and its orientation:
On a structured substrate of 1 cm²,fixed at an angle of 45?,25 µl of Au seed solution is drop cast on the substrate. The droplet moves only a few mm distance along the substrate, and eventually evaporates as time elapses.
Figure 5a shows SEM images of the center of the drop cast area with respect to the substrate angle; the experiment is carried out at cross-channel orientation. Figure 5b shows the SEM images from the central part of the drop casting area; the growth is done in in-line orientation, by maintaining a substrate angle of 45°C. SEM images clearly show that at cross channel orientation, formation of nano wire along the grooves is not obtained whereas in-line orientation favours nano wire formation along the groves.
It is noted that rod-formation occurs by the filling of the grooves in the substrate by capillary motion. In this respect, when the angled- direction is in alignment with the grooves, the action of the capillary motion is in alignment with the groove direction leading to well formed nano-rods. Conversely, where the alignment of the grooves is perpendicular to the slant angle, the capillary action leads to disconnected fragments on separate grooves, which is not a favorable configuration for nano-rod formation.
C) Angle of the substrate:
5 sets of experiments varying from 0? to 45?at 15? intervals which are 0?, 15?, 30? and 45?are carried out . Figure 6 shows that for DVD substrates, using low temperature nucleated gold solution, and temperature of 24?Cwith drop-casting based nano wire growth, a range of 30-45? angle yields nano wire growth along with maximum density and uniformly distributed nano wire formation. Figure 6 shows SEM images from different parts of the drop cast area. Figures 6c, 6d show that the central region of the drop casted area is suited for the nano wire formation.

It is observed that the maximum extent of well-connected nano-rods is formed when the angle is 45°. At zero degrees slant, the evaporation of the droplet is uniform leading to distributed non-correlated droplets. Here, while the action of capillary forces leads to the filling of the grooves, it also leads to the breaking of long fragments into smaller droplets (similar to what is observed in classical Rayleigh instability). As the angle of the substrate increases, the shape of the droplet is non-uniform leading to lesser liquid and hence thinner liquid at the top of the slant and thicker regions below. This leads to non-uniform evaporation. The nano-rod formation occurs from top to the bottom of the groove. While the action of gravity tends to make the droplets filling the groove flow downward, the capillary motion acts in the opposite direction. This configuration of forces keeps the droplets in the grooves connected, leading to the formation of continuous rod formation. With increase in substrate angle, the gravity component would dominate, while for smaller angles the capillary motion dominates. Rod-formation is achieved upon the balance of gravity and capillary motion which occurs at 45? substrate angle.
In other words, the maximum length and the density of the nano wire which is obtained by this method, depends on the substrate angle maintained during the self-assembly. Though the maximum length nanowires of 28µm is obtained is at 30?substrate angle, maximum area coverage having uniform density is observed at a substrate angle of 45?.
D) Periodicity:
In addition to DVDs, the experiment is conducted with CDs, in order to change the underlying periodicity. It is noticed that two drop-castings runs are needed for achieving results similar to DVD. Hence in principle, the method works even when the underlying periodicity is different.
For larger widths of the grooves, the particles have larger degrees of freedom leading to arbitrary orientation during crystallization. As the widths are reduced, these degrees of freedom get constrained leading to greater alignment of the particle chain fragments, which eventually coalesce coherently to form well-aligned rods.
E) Stability of the seed solution:
The Zeta potential measurement figure 9(a) and 9(b) is carried out in order to check the stability of the seed solution,, It is necessary to maintain the temperature of the seed solution from about 1°C to about 4°C and also it is necessary that the solution is not agitated and handled gently in order to prevent agglomeration. The experiment needs to be conducted in a sterile environment. Zeta potential measurements for stability of the seed solution is carried out soon after the synthesis, and subsequently for about 114 days. Multiple measurements of seed solution is done to ensure good signal to noise ratio. It is observed that Zeta potential remains almost the same soon after synthesis (of about 39mV) and after 114 days (of about35.5 mV). This is consistent with the fact that there is no change in color, or any agglomeration observed during this period..

The present invention finds application in sensing applications, biosensing applications electromechanical and electrochemical applications. One of the applications using the the present invention has been described below. Surface Enhanced Raman Spectroscopy (SERS) using the nanowires of the present invention has been observed for Sensing of Rhodamine 6G at micromolar concentration (Figure 8)
Experimental procedure for preparation of the Surface Enhanced Raman Spectroscopy (SERS) DVD substrate and SERS measurement.:
A freshly prepared seed solution of 20µml -30µml is dropcasted using a micropipette on a slanted DVD substrate of 1 cm² measure , at a slant angle of 30°-45°. After 6-7 hour from the time of completion of drop casting and drying the substrate, the substrate which comprises the Au nanowires is carefully taken out and placed on a horizontal platform. Simultaneously, a fresh solution of Rhodamine 6G solution(diluted in water) is prepared in a dimly lit room to prevent light reaction with Rhodamine molecules which may affect the performance of RAMAN signal. 20µml -30µml of freshly prepared Rhodamine solution is drop casted on the same susbtrate on which the Au seed solution is drop casted. The substrate with the Rhodamine coating is allowed to dry for about 6-7 hours in a dark room. The sample is taken out after the completion of 7-8 hours and this now forms the SERS substrate which is placed in a desiccator covered with aluminium foil until the time of usage of the coated substrate. The SERS substrate thus obtained is used for studying the Raman effect, the graph which is shown in figure 8. 514nm with 100X magnification objective lenses and 1µm -2µm spot size is used. The accumulation time of the laser illumination is 5-10seconds. The area of illumination of the laser spot is at the central portion of self assembled area of Au nanowires of the DVD

The aforesaid description is enabled to capture the nature of the invention. It is to be noted however that the aforesaid description and the appended figures illustrate only a typical embodiment of the invention and therefore not to be considered limiting of its scope for the invention may admit other equally effective embodiments.
It is an object of the appended claims to cover all such variations and modifications as can come within the true spirit and scope of the invention.