Metal Nanoparticle Based Sensors For Hydrogen Peroxide, Uric Acid And Cholesterol And The Preparation Thereof
Abstract:
An integrated enzyme-metal nanoparticle based sensor comprising integrated enzymes and
nanoscale platinum particles having size ranging from 7 to 10 nm on a conducting support
modified with biocomposite layer. A method of preparation of fabricated sensor, said
method comprising the steps of modifying a conducting support comprising
polycrystalline gold electrode with a layer of biocomposite derived from 3- (mercaptopropyl)trimethoxy silane (MPTS), encapsulating the oxidase enzymes into the network and self assembling the platinum nanoparticles on SH groups of silicate network by chemisorption.
Get Free WhatsApp Updates!
Notices, Deadlines & Correspondence
KHARAGPUR, PIN - 721 302, DIST - MIDNAPORE, STATE OF WEST BENGAL, INDIA
Inventors
1. RAJ C. RETNA
DEPARTMENT OF CHEMISTRY, INDIAN INSTITUTE OF TECHNOLOGY, KHARAGPUR-721302, WEST BENGAL, INDIA
2. JENA, BIKASH, KUMAR
DEPARTMENT OF CHEMISTRY, INDIAN INSTITUTE OF TECHNOLOGY, KHARAGPUR-721302, WEST BENGAL, INDIA
Specification
Field of the invention
The present invention relates to development of integrated enzyme-metal nanoparticle
nano-architecture based biosensors for detection of glucose, uric acid and cholesterol at
physiological level. The present invention also relates to sensing of hydrogen peroxide,
uric acid, cholesterol and glucose using nanoarchitectured sensor.
Background and prior art of the Invention
Development of a reliable method for the sensing of H202 is of great interest in many
different areas. H202 is the most valuable marker for oxidative stress; oxidative damages
resulting from the cellular imbalance of H202 and other reactive oxygen species generated
from H202 are connected to aging and severe human diseases like cancer and
cardiovascular disorder. H202 is recognized as one of the strongest oxidant in the
conversion of dissolved S02 to H2S04 is recognized as one of the strongest oxidant in the
conversion of dissolved S02 to H2S04, which is the main contributor to the acidification of
aqueous phase of the troposphere. More importantly, H202 is a product of the reaction
catalyzed by a large number of oxidase enzymes. The operating principle of the oxidasebased
electro-chemical biosensor involves the determination of enzymatically generated
H202. Highly sensitive and selective transducer is very essential for the precise
monitoring of H202 generated during the enzymatic reaction. Such sensitive transducer
can, in principle, used in the development of clinically important analytes such as uric
acid, cholesterol and glucose using the corresponding oxidase enzymes.
The carbon paste, carbon nanotube, polycrystalline Pt. Mediator and peroxidase based
electrodes have been used for the sensing of H202 (Anal. Chern. 1992,64, 1285; Langmuir
2005,21,3653; Anal. Chern. 2004, 76, 474). Deactivation of electrode surface due to the
formation of surface oxide is one of the problems with the polycrystalline Pt electrode
(anal. Chem.2003, 75, 2080). Many attempts have been made to improve the performance
of Pt based electrodes. The Pt-black, Pt-black rnicroarray and mesoporous Pt electrodes
have been used to achieve high sensitivity towards H202 (Talanta, 2006, 68, 1632, Anal.
Chem.2002, 74,5717).
The use of 111 icroelectrode array or nanoelectrode ensembles for electroanalytical
applications has several advantages over the conventional rnacroeletrodes. In principle, the
electroanalytical detection limits at micro/nanoelectrode ensembles can be much lower
than that of an analogous macrosized electrode due to the enhancement in the signal to
noise (SIN) ratio (AnaI.Chem.1987, 59,2625).
Quantification of cholesterol uric acid and glucose in clinical sample is very essential as
the elevated concentration leads to life threatening diseases. The normal total cholesterol
level in serum is 3 to 6.5 mM (Anal. Chem.1993, 65, 3258). Most of the cholesterol (2/3)
in serum exists in the form of ester. The normal uric acid level in serum range from 240 to
520 ~lM and in urinary excretion 1.4 to 404 mM (J. Assoc. Off. Anal. Chem. 1987, 70, I).
The physiological level of glucose is 3-8 mM (The New England J. Medicine 2005, 353,
1454). First and second generation biosensors have been proposed for the precise
estimation of these analytes. In both the cases, oxidase enzymes are used. In the first
generation biosensors, the estimation of enzymatically generated hydrogen peroxide is
considered to be the direct measure of the actual concentration of the analyte. On the
other hand, oxidase enzymes along with a suitable mediator, which shuttles the electron
transfer between the enzymes and electrode surface, is used in the second generation
biosensors. The major problem associated with the mediator-based biosensors is the lack
of long term stability due to the leaching of mediator from the electrode surface.
Various methodologies have been used for the electrochemical sensing of H202 uric acid
and cholesterol. I-b02 can be conveniently quantified electrochemically by its reduction or
oxidation. Enzymes and redox mediators that catalyze the oxidation/reduction I-b02 have
been conventionally used. Peroxidases, Prussian Blue, carbon nanotube (CNT),
mesoporous Pt microelectrodes and metal particle-based electrodes are widely used
(AnaI.Chem.2004, 76,474; Anal. Chem.2002, 74, 1322; Biosensor Bioelectron. 2009,
24,3264). Detection of H202 by its reduction is susceptible to interference from oxygen.
On the other hand oxidation of at more positive potential invites interference from other
small electroactive molecules. One of the serious problems in the electrochemical
detection of H202 is the interference due to easily oxidizable analytes commonly available
in the samples. Mitigation of interference without compromising the sensitivity is a
challenging task. The electrode that can catalyze the oxidation of H202 at less positive
potential (DAV) is desired for the sensing of H202. The redox mediator-bases electrodes
are not stable. presumably due to the leaching of the mediator from the electrode surface.
Lyon and Stevenson achieved the lowest detection limit of 8 pM H202 using chemically
,.'
activated redox mediator (Anal. Chem. 2006, 78, 8518). However, this method requires
diffusional redox mediator and enzyme horseradish peroxidase. The disadvantage of this
method is that the redox mediator easily absorbs on the electrode surface and hence
deactivation of the electrode is inevitable. A detailed survey of literature on the
electrochemical sensors for H202 is given in Table I.
Jyh-Myng Zen and Jen-Sen Tang (Anal. Chem. 1995, 67, 1892) modi fled the glassy
carbon electrode by Nafion !RU2_'Pb,07. x (ruthenium oxide pyrochlore) and detected uric
acid by Osteryoung square -wave voltammetry. The linear range obtained with this·
electrode was from 7.5 x 10.5 to 5x 10-7 M and the limit of detection was 1.1x 10-7M The
major concern with this electrode is that it needs to be operated in acidic pH (pH I). Cai
et al. utilized electrochemically activated carbon paste electrode for the detection of uric
acid (Talanta, 1994, 41,407). This electrode showed linear response from 3x 10-8 to
2Ax I0-4 M and the limit of detection was 1.2x 10-8 M. However, the preparation of the
electrode is rather time consuming and the linear range was not in the diagnostic range of
the physiological sample. Yu et al. modified glassy carbon electrode with polyglycine and
examined the voltammetric response towards uric acid and ascorbic acid (0.30 mY vs.
SCE) and increased the response current of uric acid after redox reaction. Although the
electrode could distinguish the voltammetric response, the linear range was out of normal
physiological level. Nakaminami et al. developed enzyme based uric acid sensor using
redox polymer and enzyme uricase. This method is based on the mediated oxidation of
uric acid by the enzyme. The analytical parameters such as the limit of detection, linear
range etc. was not reported (Anal. Chem. 1999, 71, 1928. Raj and Ohsaka utilized selfassembled
monolayer of heterocyclic thiol for the non-enzymatice detection of uric acid
(J. Electroanal. Chem. 2003, 540, 69). This electrode could sense uric acid in the presence
of ascorbic acid. However, the lowest concentration detected was 1 x J 0.6 M.
Uric acid and cholesterol are traditionally detected by time consumtng colorimetric
methods. These methods require color indicating reagents and enzymes. Uric acid has
been quantified in real samples using chemically modified electrodes and redox mediator
based electrodes. Carbon-based electrode with preconcentration procedures has been
widcly used for the voltammetric sensing of uric acid. Sol-gel derived ceramic film with
redox mediator has been used for the sensing of uric acid by the oxidation of uric acid
lAnaI. Chem.2002, 74, 5734-5741]. Redox mediator-based biosensors have been llsed for
the sensing of uric acid (Anal. Chern. 1999,71,1928-1934; Anal. Chern. 1999,71,4278-
4283). ruj ishima and co-workers have reported the voltammetric sensing of uric acid
using boron doped diamond electrode; this electrode requires as large as 0.9 V (SCE).
Nanostructured carbon fiber electrode has been used for the sensitive detection of uric acid
fast sweep voltammetry (AnaI.Chem.2000, 72, 1576-1584). Selectivity and sensitivity is a
major problem with unmodified electrodes. The enzyme based electrodes highly selective
and sensitive in the determination of uric acid.
Unlike uric acid, direct oxidation of cholesterol is not possible on conventional electrodes.
Traditionally, sensing of cholesterol has been performed with enzymes cholesterol oxidase
and cholesterol esterase using redox mediators (Biosensors and Bioelectronics 2008, 23,
1083-1100 and the references cited therein). Lack of long term stability, selectivity and
poor sensitivity are the draw backs with the existing sensors reported in the literature.
Lu et at. (200 I - 818000) disclosed the development of oxidase base sensor for the
sensing of uric acid using oxidase enzyme and metal doped carbon composite electrode.
Hydrogen peroxide is generated during the enzymatic reaction of uric acid with enzyme.
Enzymatically gen'erated hydrogen peroxide was electrochemically detected by its
reduction.
Shen et al. (1999 - 295400) developed non-enzymatic electrochemical sensor for the
sensing of uric acid using water soluble redox mediator and polymer. Stability, lowest
detection Iim it are not given.
Hsiung et at, (2005-211679) disclosed the fabrication of enzyme based sensor for the
sensing of uric acid using enzymes catalase, uricase and redox mediator ferrocene
carboxylic acid. The amperometric current due to the reduction of enzymatically
generated hydrogen peroxide was used to quantify the concentration of uric acid in the
sample. Polyacrylamide was used to bind the enzyme and redox mediator.
Chuang et al (2007 - 782517) disclosed a biosensor, a biostrip and a manufacture method
of determination of uric acid by a non-enzymatic reagent. The biosensor of the invention
comprises at least a biostrip and a non-enzymatic reagent for the determ ination of uric
acid. The biostrip has at least two working electrodes and at least one reaction zone. The
non-enzymatic reagent is disposed in the reaction zone' on the biostrip where a tetrazolium
salt and an active electron mediator are immobilized. The reaction zone is used for a
sample to initiate the oxidation-reduction rcaction of the tctrazolium salt and then,
according to the oxidation - reduction reaction, with the active electron mediator and the
two working electrodes an electronic signal is transmitted to the sensor to produce a
corresponding micro-current intensity which is in turn calculated by thc sensor to reflect
the uric acid level in the sample.
Kumar et al. disclosed (US 7175746) the development of polymer based cholesterol
biosensor using potassium ferricyanide as a mediator. The enzyme cholesterol oxidase
was physically adsorbed on the electrode surface and it has a response time of 30 s. The
real sample analysis has not been performed with the proposed sensor. The major concern
with this biosensor is that it may not be used directly for the quantification of total
concentration of cholesterol in real sample analysis, as it uses only cholesterol oxidase. In
serum, 2/3 of cholesterol exists as ester and quantification of total cholesterol in serum
requires the enzyme cholesterol esterase.
US 7257837 disclose the development of cholesterol sensor using cholesterol oxidase, solgel
silicate in the presence and absence of redox mediator. The enzyme and redox
mediators were encapsulated in to the silicate net work and cast on the conducting
substrate. In the absence of redox mediator, cholesterol was detected at the potential of
0.9 V by measuring the current for the oxidation of enzymatically generated hydrogen
peroxide. Interference due to other small molecules was noticed, due to the high
overpotential. In the prescnce of redox mediator, the detection potential was decreased to
0.4 V. This sensor has the response time of 30-90 s and was reused at least for five times.
The utilization of the sensor for real sample analysis has not been demonstrated and the
sensor may not be used for the quantification of total cholesterol in real samples.
US 6117289 describe a cholesterol sensors fabricated using cholesterol oxidase,
cholesterol esterase. redox mediator and a surfactant. The sensor operates at the potential
of 0.5 V. The reaction layers contain the enzymes and the redox mediator. The response
time of the sensor is 9 min and the utilization of surfactant may interfere in the real sample
analysis. Real sample analysis has not been described.
US 6071392 disclose a cholesterol sensor which comprises an electrode system having a
working electrode and a counter electrode formed on an electrically insulating base plate.
The electrode coating layer comprising water-soluble cellulose derivatives and saccharides
for covering the electrode. The reagent layer contains the enzymes cholesterol oxidase
and cholesterol esterase and surfactant. The configuration of the sensor eliminates
impairment of sensor response due to electrode degeneration caused by invading
surfactant into the electrode system. The sensor was operated at the potential of 0.5 V.
US 62146] 2 disclose a method for the quantitative determination of cholesterol using a
carbon working and counter electrodes. The reaction reagent contains cholesterol
dehydrogenase, electron transfer mediator, nicotinamide adenine dinucleotide (NAD) and
diaphorase. NAD is reduced to NADH during the enzymatic reaction; NADH oxidized to
NAD by the electron transfer mediator. The concentration of the reduced form of the
electron transfer mediator is directly proportional to the concentration of cholesterol. The
reaction ofNADH with mediator was catalyzed by another enzyme diaphorase. All these
reactions are favorable only at pH>9. The reaction is very slow at lower pH. The
operating potential of the sensor was 0.5 V. For real sample analysis, high pH is not
desirable.
Apart from the above prior arts, others also discloses the sensing of cholesterol using one
or more redox mediators, surface active reagents (US patents 6966977, 1239048, 5695947,
6451372,6821410,5695947,1347292,2007-0158189, 6117289, 2002-084937,1318396,
6117289,7267837, 1239048,6342364). However, the standing difficulties are: (i) require
large overpotential, (ii) lack of selectivity, (iii) suitability of the sensors for real sample
analysis etc.
US 2004/0077844 (US'844 in short) relates to fabrication and arrangement of
nanoparticles into one-dimensional linear chains achieved by successive chemical
reactions, each reaction adding one or more nanoparticles by building onto exposed,
unprotected linker functionalities. Nanoparticle spheres are functionalized in a controlled
manner in order to enable covalent linkages. Functionalization of nanoparticles is
accomplished by either ligand exchange or chemical modification of the terminal
functional groups of the capping ligand. The method tor assembling nanoparticles in a
controlled fashion comprises the steps of: providing a plurality of nanopariicles; providing
a plurality of ligands. each of the ligands comprising at least one linker arm; attaching at
least one Iigand to each of the plurality of nanoparticles; and reacting at least one pair of
the linker arms to ronn an assembly of nanoparticles.
Fadime et al (Sensors 2009) relates to a amperometric cholesterol biosensor with
immobilization of cholesterol oxidase on electrochemically polymerized polypyrrole
polyvinylsulphonate (PPy-PYS) films has been accomplished via the entrapment
technique on the surface of a platinum electrode. Fadime et al. further reports that the
immobil ization of cholesterol oxidase onto PPy-PYS film via an entrapment procedure for
determination of free cholesterol. Fadime et al. used enzyme immobilized polymer based
working electrode, Pt counter and Ag/ AgCI reference electrodes. Immobilization of
cholesterol oxidase was carried out by the physical entrapment approach. The
concentrations of pyrrole and polyvinylsulphonate were 0.1 M, respectively. The
monomers were electrochemically polymerized on the electrode surface in the presence of
cholesterol oxidase. Amperometric response studies were also carried out in phosphate
buffer. Operational stability, storage stability, pH and temperature were determined via
application of +0.4 Y with respect to Ag/AgCI electrode to detect oxidation current of
H202• The study shows that uric acid has no interfering effects on the analysis of
cholesterol. But interfering effects of ascorbic acid, paracetamo] and glucose on the
analysis of cholesterol were observed. These interferences were almost removed by
dilution of solution.
Further, these abovementioned literatures describe the electrochemical sensing of
cholesterol by enzymatic methods. Most of them involve the use of mediators. The
enzymatically generated hydrogen peroxide was detected either by its oxidation or by its
reduction. Oxidation of hydrogen peroxide at more positive potential invites interference
form other analytes co-exist in the sample. Oxidation at less positive (<<;0.45V) is
preferred without mediator. The second approach is by the reduction at negative potential:
this approach is susceptible for interference due to dissolved oxygen. In many of the
literature, the real sample analysis has not been performed to validate the utility of the
sensor for the analysis with real systems. Stability, sensitivity and response time of the
sensors need significant improvement.
The sensor described in the aforementioned prior art does not deal with the real sample
analysis and cholesterol ester. It describes the use of conventional polymer and cholesterol
oxidase for the sensing of commercially available cholesterol (not total cholesterol in
serum) which may not be applicable for real samples.
Furthermore, ascorbic acid, paracetamol and glucose severely interferes the measurement.
It is mentioned in Fadime et al. that dilution of the sample can remove the interference.
Dilution of real sample is not recommended.
Hence there is need for improved method for use in real samples that do not suffer from
interference with other analytes and other drawbacks of the prior art and yet provide
improved stability, sensitivity and fast response time of the sensors.
Object of the invention
The principal object of the present invention is to provide a biosensor by integrating Pt
nanoparticles and oxidase based enzymes with a conducting substrate.
Another object of the present invention is to provide an integrated enzyme-metal
nanoparticle nano-architecture for the sensing of uric acid, cholesterol and glucose at
physiological level.
A further object of the present invention is to provide a fabricated biosensor that detects
hydrogen peroxide at sub nanomolar level without any interference.
Yet another object of the present invention is to provide a fabricated biosensor that detects
uric acid/cholesterol/glucose without any redox mediator at physiological level.
Yet another object of the present invention is to provide a sensor that does not require
sophisticated equipments.
Yet another object of the present invention is to provide a sensor that docs not suffer from
interference from other electroactive interferents present in the real samples.
Summary of the invention
According to one aspect of the present invention. there is provided an integrated enzymemetal
nanoparticle based sensor comprising integrated enzymes and nanoscale platinum
particles h<:iving size ranging from 7 to 10 nm on a conducting support modified with
biocomposite layer.
According to another aspect of the present invention. there is provided a fabricated sensor
prepared by the method comprising the steps of
I. modifying a conducting support comprising polycrystalline gold electrode
with a layer of biocomposite derived from 3-(mercaptopropyl)trimethoxy
silane (MPTS);
II. encapsulating the oxidase enzymes into the network;
iii. self assembling the platinum nanoparticles on -SH groups of silicate
network by chemisorption.
Brief Description of the accompanying Figures
Figure 1 illustrates the fabrication ofnano-architectured biosensor.
Figure 2 displays the amperometric trace obtained for the oxidation of H202 on the Pt
nanoparticulate based architecture.
Figure 3 illustrates the interference free amperometric sensing of H202• Ascorbic acid,
uric acid and paracetamol (0.1 mM each) were injected at regular interval.
Figure 4(A) shows the amperometric response obtained for the biosensing of uric acid
with the nano-architectured biosensor.
Figure 4(8) shows corresponding calibration plots.
Figure 5(A) shows the amperometric response obtained for the biosensing of cholesterol
cster at the nano-architectured biosensors.
Figure 5(8) is the corresponding calibration plots.
Figure 6(A) illustrates the amperometric response obtained for the biosensing of glucose
with the nanoarchitectured biosensors.
Figure 6(8) shows the corresponding calibration plot
Detailed Description of the Invention
The present invention relates to the development of integrated nano-architectured sensors.
In the present invention. metal based nanoparticulates have been self assembled and
encapsulated into a network.
According to one embodiment of the present invention, there is provided an amperometric
sensor for the sensing of clinically important analytes such as hydrogen peroxide, uric
acid, cholesterol and glucose is developed using nanosized Pt particles and oxidase based
enzymes.
A biosensor is developed by integrating Pt nanoparticles and oxidase based enzymes with
a conducting substrate. The enzymatic reaction of clinically important analytes with the
oxidase enzymes generates hydrogen peroxide. The amount of enzymatically generated
hydrogen peroxide is directly proportional to the actual concentration of the analyte
present in the sample. Enzymatically generated hydrogen peroxide is quantified
amperometrically at the potential of 0.45 V (Ag/AgCl) by the Pt nanoparticles on the
sensor. In the case of uric acid biosensor, enzyme uricases is used whereas enzymes
cholesterol oxidase and cholesterol esterase are used in the development of cholesterol
biosensor; glucose oxidase is used in the case of glucose biosensor. Real samples have
been tested to val idate the method.
According to the present invention, there is provided a sensor comprising a three
dimensional (3D) silicate network on a solid support used for the integration of enzymes
and Pt nanoparticle. Enzyme and Pt nanoparticle integrated 3-D network on a solid
support has been developed for the sensing of total cholesterol and uric acid. The enzymes
are encapsulated into the 3-D network and nanoparticles are chemisorbed onto the thiol
groups. Such integration of Pt particle and enzyme(s) (uricase, glucose oxidase,
cholesterol oxidase, cholesterol esterase) for the fabrication of electrochemical sensors has
not been disclosed in prior art. The distribution of metal nanoparticle throughout the
network is an added advantage; it favors the facilitated reaction.
Different metals that can be used as conducting support are polycrystall ine Au. Pt, Pd and
Cu.
The present invention teaches a sensor having both cholestcrol oxidase and cholesterol
esterase. It is well known that more than 80% of cholesterol in real sample (serum) exists
in the form of ester, which cannot be detected only by cholesterol oxidase. Cholesterol
ester should be hydrolyzed to cholesterol by another enzyme cholcsterol esterase. The
hydrolyzed product (cholesterol) can undergo enzymatic reaction with cholesterol oxidase
and generates hydrogen peroxide, which can be detected electrochemically. For the
measurement of total cholesterol in serum sample, the sensor should have both cholesterol
oxidase and cholesterol esterase.
The present invention teaches the integration of enzyme(s) (cholesterol oxidase and
cholesterol esterase in the case of cholesterol sensor) and nanoscale Pt particles on a
conducting support. Such integration of Pt nanoparticles and enzymes of any kind has not
been found either in the literature or in the prior art. The analytical application of the
sensor is demonstrated with real sample analysis. The results are authenticated/validated
with clinical laboratory measurements (as given in the Table 2). The sensor developed in
the present invention could detect (i) hydrogen peroxide at sub-nanomolar level and (ii)
cholesterol, uric acid and glucose at well below the physiological leve!' It has wide linear
range and does not suffer from the interference due to other analytes.
Dilution ofthc sample is not required in the present invention and more importantly it has
the response time of 2s.
further aspect of the present invention lies in the development of an integrated enzymemetal
nanoparticlc nano-architecture for the sensing of hydrogen peroxide at subnanomolar
level and sensing of uric acid, cholesterol and glucose at physiologicalleve!.
Hydrogen peroxide is detected in range from 0.1 nM to 1.4 mM. Uric acid is detected 100
nM - 200 ~M, cholesterol is detected at 0.5 /-lM - 12 mM and glucose is detected at a
range of 10 nM - 20 mM.
The detection range can be varied by changing the enzyme loading in the sensor. The
sensor can d~tect nanomolar levels (well below the physiological level) of glucose and
uric acid and sub-micromolar level in the case of cholesterol.
Further aspect of the invention involves the fabrication and characterization of the
sensor/biosensor by using a conducting support modified with a biocomposite layer to
form a 3D silicate network.
Yet another aspect of the invention involves the formation of a self assembled group of the
silicate network by chemisorptions comprising platinum nanoparticulates.
Yet another aspect of the present invention features the experimental testing with
commercial and real samples.
The present invention provides a fabricated nano-architectured sensor which is stable for
II days when stored in phosphate buffer solution of pH 7.2 having only 9% decrease in
the initial response after 15 days.
The operation stability of the samc electrode when subjected to 20 repetitive
measurements for a period of 24 hI', the coefficient of variation in the current is only
0.12%.
Advantages of the present invention
The present invention unlike the existing system of the prior art has the following
advantages.
I. The present sensor is advantageous in view of sensing of hydrogen peroxide at
sub-nanomolar level without any interference.
2. Biosensing of uric acid/cholesterol/glucose without any redox mediator at well
below the physiological level and show linear response for wide concentration
range.
3. The present invention does not require sophisticated equipments.
4. The present invention also does not suffer from interference from other
electroactive interferents present in the real samples.
The invention is now defined by way of non-limiting illustrative examples:
Example I
Fabrication and characterization of sensor/biosensor
The fabrication of this sensor involves the following procedures: (i) the conducting
support (polycrystalline Au electrode) was first modified with a layer of biocomposite
(enzyme-sol-gel 3-D silicate network) derived from 3-(mercaptopropyl)trimethoxy silane
(MPTS). This network has plenty of -SH functional groups. The oxidase enzymes were
encapsulated into the network; (ii) Pt nanoparticulates have been self-assembled on the -
SH groups of the silicate network by chemisorption.
The size and morphology of the nanoparticulates on the silicate network have been
examined with field emission scanning electron microscope (FESEM) measurement. The
nanoparticulates are randomly distributed throughout the silicate network on the electrode
surface and have the size distribution between 7 and 10 nm. FESEM images confirm the
encapsulation of enzyme inside the silicate network. FESEM measurement was performed
with lEOL lEM 6700F microscope. The Au coated glass slides were first functionalized
with MPTS network and then Pt nanoparticles were immobilized by chemisorption (as
depicted in Figure I).
Example 2
The electrochemical measurements were performed with computer controlled CH 1643
electrochemical analyzer. Electrochemical cell consists of three electrodes. Working:
nano-architectured electrode; auxiliary: Pt wire; reference Ag/ AgCI saturated with 3 M
NaCI. The colloidal Pt nanoparticulates were synthesized according to the reported
procedure (Chem. Commun. 2005, 2972 - 22974) with little modification. Briefly, 25 ml
aqueous solution of H2PtCI6 (0.02 mM) and I mM glucose was mixed and stirred for 2
min. Then the pH of the solution was adjusted to 8 with NaOH. A 400 III of aqueous
NaBH4 (0.05 M) was then added drop wise to the stirred solution: the stirring was
continued for 15 min. The formation of nPt was followed by UV-visible spectral
measurement. The color of the solution changed to grey-brown and the peak at 200-300
nm disappeared upon the addition of reducing agent, indicating the reduction of the PtCi6
2
-
ions to metallic Pt.
The MPTS sol was prepared by the dissolving MPTS, methanol and water (as 0.1 M HC!)
in the molar ratio of I:3:3 and stirring the mixture vigorously for 30 minutes. For the
fabrication of biosensor, the MPTS sol-enzyme biocomposite was prepared by the
following procedure: first the MPTS sol was prepared by taking 24flL Hel (0.] M in 2.ml
water and stirring the mixture vigorously for 30 min. Then 0.5 ml MPTS sol was mixed
with 0.5 ml of enzyme (uricase) solution (20 mg/ml in 5 mM phosphate buffer solution of
pH 8) and stirred for 2-3 min for the encapsulation of enzymes into the silicate network.
For the fabrication of cholesterol biosensor, cholesterol, esterase (10 mg/ml) and
cholesterol oxidase (20 mg/m!) were mixed with MPTS sol as described earlier. The
resulting sol-gel biocomposite was stored at 4°C.
The cleaned polycrystalline Au electrode was first soaked in MPTS sol-enzyme
biocomposite for 20 min for the spontaneous adsorption of enzyme encapsulated sol-gel
net-work on the electrode surface. The nPts were then self-assembled on the free thiol
groups of the enzyme encapsulated sol-gel network at 4°C by soaking the enzyme
encapsulated silicate network modified electrode (Figure I).
Example 3
Amperometric sensing ofH202
Figure 2 displays the amperometric trace obtained for the oxidation of H202 on the Pt
nanoparticulate based architecture. The electrode was polarized at 0.45 V and aliquots of
H202 were injected into a stirred phosphate buffer solution (PBS). Fast and stable
response was obtained upon every injections; the response time was 2s. The sensitivity
and the experimental limit of detection (SIN = 9) were found to be 1.82±0.O I nA/nM and
0.1 nM respectively. The 3D nanoarchitecture design on the electrode surface allows the
facile access of the reactant to the catalyst site and the rapid removal of the product by
diffusion to solution from the film.
The elimination of interference due to easily oxidizable compounds present in the
physiological system is a real challenge in the voltammetric detection of H202 Ascorbic
acid; uric acid and paracetamol are the major interfering agents present in the
physiological system. To examine the performance of nano-architectured electrode in the
presence orthe interferents, amperometric response of the electrode toward H202 has been
tested in the presence of ascorbic acid, uric acid and paracetamol. The amperometric
responsc of the nPt electrode was first registered by injecting H202 (30 M) into a stirred
supporting electrolyte and an aliquots of ascorbic acid, uric acid and paracetamol (0.1 mM
each) have been injected subsequently at regular interval (Figure 3). Interestingly, no
observable change in the steady state current for H202 was noticed upon the addition of
these interferents, indicating that the present electrode can be successfully used for the
measurement of H202 without any interference. The sensor is highly stable, no change in
the initial response was obtained for II days and 7% decrease in the current was noticed
after 15 days (same electrode).
Example 4
Measurement of H202 in rainwater
The analytical utility of the Pt nano-architectured electrode was demonstrated by
measuring the concentration of H202 in rainwater. Four samples were collected and
analyzed amperometrically. Stable amperometric response was obtained upon every
injection. Addition of standard H202 solution (standard solution means calibration plot
was made with known concentration) showed linear relationship between the
amperometric current and concentration of H202. The total concentration of H202 in the
rainwater was in the range of 150 to 170 nM (Table 1). The recoveries for the spiked
H202 (spiked solution means after the addition of rainwater, known concentration of H202
was spiked and the recoveries were calculated from the response) sample wcre 92-94%.
Table 1: Determination of H202 in rainwater using nano-architectured platform a
Sampleb
2
Original value Total concentration
(nM) (nM)
1.5 150
1.66 170
1.68 186
1.66 166
3
4
a three independent amperometric measurements were made with each samples.
b collected in the months of August 2007 (samples I and 2) and January 2008 (samples 3
and 4).
Example 5
Biosensing of uric acid, cholesterol and glucose.
The oxidase enzymes catalyze the oxidation of variety of analytes in the presence of
oxygen; H202 is generated during the enzymatic reaction. Since the amount of
enzymatically generated H202 is proportional to the analyte concentration, the H202
sensitive electrodes can be used for the measurement of analyte present in the biosample.
The biosensor was fabricated by integrating the oxidase enzyme and nPts with the silicate
network (Figure I) . .In the case of cholesterol, cholesterol esterase and cholesterol oxidase
were used; cholesterol esterase hydrolyzes cholesterol ester into cholesterol and
cholesterol oxidase converts the enzymatically generated cholesterol to 4-cholesten-3-one.
The enzyme encapsulated into the network efficiently catalyzes the oxidation of uric acid /
cholesterol! glucose in the presence of oxygen. Enzymatically generated H202 sensitive
electrodes can be used for the measurement of analyte present in the biosample. The
biosensor was fabricated by integrating the oxidase enzyme and nPts with the silicate
network (Figure 1).
In the case of cholesterol, cholesterol esterase and cholesterol oxidase were used;
cholesterol esterase hydrolyzes cholesterol ester into cholesterol and cholesterol oxidase
converts the enzymatically generated cholesterol to 4-cholesten-3-