Counting the number of enzymes immobilized onto a nanoparticle-coated electrode
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To immobilize enzymes at the surface of a nanoparticle-based electrochemical sensor is a common method to construct biosensors for non-electroactive analytes. Studying the interactions between the enzymes and nanoparticle support is of great importance in optimizing the conditions for biosensor design. This can be achieved by using a combination of analytical methods to carefully characterize the enzyme nanoparticle coating at the sensor surface while studying the optimal conditions for enzyme immobilization. From this analytical approach, it was found that controlling the enzyme coverage to a monolayer was a key factor to significantly improve the temporal resolution of biosensors. However, these characterization methods involve both tedious methodologies and working with toxic cyanide solutions. Here we introduce a new analytical method that allows direct quantification of the number of immobilized enzymes (glucose oxidase) at the surface of a gold nanoparticle coated glassy carbon electrode. This was achieved by exploiting an electrochemical stripping method for the direct quantification of the density and size of gold nanoparticles coating the electrode surface and combining this information with quantification of fluorophore-labeled enzymes bound to the sensor surface after stripping off their nanoparticle support. This method is both significantly much faster compared to previously reported methods and with the advantage that this method presented is non-toxic.
KeywordsGold nanoparticles Immobilized enzyme Enzyme quantification Microelectrode Electrochemical stripping Glucose oxidase
Since Clark and Lyons developed the first enzyme-based biosensor for monitoring glucose in 1962 , there have been a tremendous variety of enzyme-based biosensors created for different applications such as food industry, pharmacology, medicine, environmental analysis, and chemistry [2, 3, 4, 5, 6, 7]. Electrochemical detection of enzymatic product formation is a widely used approach in many biosensor systems to indirectly probe an analyte serving as the enzyme substrate and that is not electroactive and therefore not directly detectable using electrochemistry [8, 9, 10, 11]. Common electrode materials for constructing enzymatic sensors include carbon and different noble metals, e.g., platinum and gold. It has previously been shown that immobilizing enzyme on a high curvature surface is beneficial to retain enzymatic activity by minimizing changes in tertiary structure of the enzyme upon attachment and preventing denaturation [12, 13]. In order to achieve high curvature topology at the electrode for enzymes to bind, surfaces can be modified with various nanomaterials such as nanotubes and nanoparticles (NPs) . Additionally the immobilization of nanostructures will increase the surface area of the electrode and thereby allow a higher enzyme loading. The approach of modifying electrode surfaces with metal nanoparticles has been extensively used [15, 16, 17, 18]. Gold is a material often exploited, due to its high biocompatibility and its enhanced activity to oxidize/reduce electroactive reporter molecules, e.g. hydrogen peroxide from enzymatic reactions involving the analyte [19, 20]. The nanoparticles can be deposited onto the electrode surface by several methods including drop casting a solution of nanoparticles [21, 22], seed-mediated growth method [16, 23], or electrodeposition from a gold ion containing solution [24, 25]. For biosensor applications in biological systems, the most essential parameters to optimize are selectivity and sensitivity due to the challenge of detecting just the analyte when probing in a complex biological matrix and to ensure analytes present at physiological very low concentration are detectable. In addition, for certain applications, sensor temporal resolution needs to be fast enough for detection of biologically relevant fluctuations of analyte on the time scale that these events occur. The sensor size needs to be considered for optimizing desired spatial resolution and for in vivo probes to minimize tissue damage upon insertion. Deeper investigations of how enzymes interact with the electrode surface upon immobilization are of great importance when constructing a biosensor and in optimizing the sensor performance to meet the properties needed for the sensor application. Our research has been directed towards the development of biosensors for analyzing rapid release of non-electroactive neurotransmitters from secretory cells. We recently showed that a thin enzyme coverage, preferably a monolayer at the surface of an electrode with deposited gold nanoparticles (AuNPs), is a critical factor for constructing a biosensor that can push temporal resolution from the sub-second limit of conventional probes down to the millisecond time scale . Other biosensors for in vivo analysis have been based on the attachment of enzyme multilayers to improve sensitivity and additionally a protective film, such as nafion®, or a size exclusion membrane to meet the requirements needed for selectivity for in vivo analysis . A thicker layer of enzyme coating affects the sensor temporal resolution since the reporter molecule that is produced after the enzymatic catalysis reaction has a longer distance to travel to be detected by the electrode compared to the minimal distance needed to diffuse when the enzyme coating is limited to a monolayer. On the other hand, more enzymes present at the sensor surface produce a larger amount of detectable product that enhances the sensor sensitivity. So basically, this comes down to a tradeoff between sensor sensitivity and temporal resolution . Thus, depending on the specific sensor application, one or the other of these two parameters needs to be prioritized at sensor design and will determine what kind of enzyme surface interactions that needs to be optimized for enzyme immobilization. Upon attachment to a surface, enzymes can either maintain the macromolecular shape or to various degrees alter the tertiary structure. If any level of denaturation is induced, this can result in alterations of enzymatic activity and selectivity when enzyme binds to a surface, and therefore, it is important to characterize the enzyme when immobilized to the specific surface material that will be used in sensor fabrication. The characterization of immobilized enzymes is especially important for biosensors that require multiple sequential enzymes in order to produce an electro-active detectable reporter molecule, as the enzymatic activity in bulk might differ significantly for one or more of the enzymes when attached. Therefore the theoretical optimal ratio between enzymes may vary greatly when co-immobilized at the sensor surface and hence to achieve optimal sequential enzymatic activity, analysis should be performed after adsorption of various ratio of enzymes added to the surface .
To functionalize the electrode surface with nanoparticles, protocols that supply coatings with desired nanoparticle size and surface coverage are central to attain. In this work, we have optimized conditions for electrodeposition of AuNP at the surface of a glassy carbon (GC) electrode. To characterize enzyme adsorption at the AuNP surfaces of the electrodes, it is important to correlate the quantitative results of enzyme coating to the surface analysis of each individual electrode. Therefore, after AuNP deposition to perform a careful characterization of the nanoparticle size and population density at the sensor surface scanning electron microscopy imaging analysis is commonly used [23, 29]. The Compton group also developed an alternative method that electrochemically measures the gold surface at the modified electrode and by electrochemically stripping off the AuNPs, the average size and density of the AuNPs that had been deposited on the electrode surface was determined . Previous methods for quantification of immobilized enzyme at a nanoparticle structured electrode surface, enzymes were labeled with a fluorescent tag before attaching to AuNPs at the surface of a carbon electrode and then by dissolving the AuNPs in KCN the freed enzymes in solution were quantified using fluorimetry [30, 31]. As characterizing the AuNPs size and density at the sensor surface first with SEM imaging and then by dissolution of AuNPs in KCN is time consuming and a toxic process, we therefore developed a new method that is significantly faster and safer to work with. In this new method, we combine the determination of the size and density of electrodeposited AuNPs at the surface of a GC electrode by an electrochemical stripping procedure. This was followed by quantification of fluorophore labeled enzymes that were immobilized to the AuNP coated GC electrode and subsequently released after the AuNP stripping procedure using fluorimetry, which directly provides information on the number of immobilized enzymes at the AuNP surface in a non-toxic method. The presented method greatly facilitates the characterization of enzyme-based AuNP structured electrochemical biosensors. Both with the goal of finding the optimized conditions needed for enzyme monolayer coverage for design of a sensor with high temporal resolution, as well as for identifying the amount of enzyme immobilized when several layers are desired as described above. This method will also work for quantifying several different enzymes immobilized simultaneously since each type of enzyme can be labeled with different fluorescent tags .
Materials and methods
AlexaFluor 488 protein labeling kit was purchased from Invitrogen (Carlsbad, CA). Glucose oxidase from Aspergillus niger (type VII), sodium phosphate dibasic, potassium phosphate monobasic, sodium chloride, sodium bicarbonate, sulfuric acid, copper sulfate, acetic acid, sodium acetate, and ferrocene methanol was purchased from Sigma-Aldrich (St. Louis, MO). All reagents used where of reagent grade and used as received. Deionized water (resistivity ≥ 18 MΩ cm) was used in all experiments.
Electrochemical measurements were performed using a three-electrode system with a computer-controlled 1000C Series Multi-Potentiostat from CH Instruments, USA. For all experiments a 3-mm in diameter GC electrode (CH Instruments, USA) was used as working electrode, a platinum electrode as auxiliary electrode and a saturated Ag/AgCl was used as reference electrode unless otherwise stated. All potentials are reported relative to the NHE electrode potential. Prior of use the GC electrode was polished with an alumina slurry (0.05 μm particles) according to the protocol provided from CH Instruments, USA. After polishing, the electrodes were sonicated in deionized water for 10 min in an ultrasonic bath and extensively rinsed in DI water. All electrodes were tested in 1 mM ferrocene methanol by performing cyclic voltammetry between 0 and 0.8 V at 0.1 V s −1 and each voltammogram was evaluated in order to verify the electrodes were well-functioning prior to each experiment.
AuNP functionalization at the GC electrode surface
Electrodes were functionalized with AuNPs by using an electrochemical deposition protocol similar to Finot et al.  with minor alterations in the gold chloride concentration and deposition time used in order to optimize AuNP size and electrode coverage. Briefly, the AuNPs were electrodeposited onto the GC electrode surface using a 1-mM HAuCl4 solution in 500 mM H2SO4. Electrodeposition was performed by applying a potential of + 1.4 V for 10 s followed by a potential of − 0.4 V for 24 s. After deposition, the electrode was extensively rinsed with deionized water.
Electrochemical measurements of AuNP density and size at the electrode surface
An electrochemical linear sweep method adapted by Finot et al. was used in these experiments for the determination of the total surface area of the AuNPs coating the electrode surface . Briefly, a constant potential of + 1.7 V (vs. NHE) was applied to the electrode surface placed in a 500-mM H2SO4 solution and was held constant for 5 s before sweeping the potential at 0.1 Vs−1 down to + 0.8 V. .The resulting reduction peak at approximately + 1.1 V was recorded and integrated to determine the associated charge detected from the induced redox reaction at the AuNP surface. A variety of coefficient values for relating the total charge transfer from the reduction of a monolayer of oxides at the gold electrode surface to the total gold surface area at the electrode have been reported by different research groups, e.g., a constant corresponding to 543 μC cm−2 as proposed by Habrioux, A., et al.,  400 μC cm−2 proposed by Trasatti, S. and O. Petrii, , and 450 μC cm−2 was suggested by Tan. et al. . In this study, a factor of 489 μC cm−2 as determined by Finot et al. was used in this work as a similar method for electrodeposition of the AuNPs to the electrode surface was applied . The accuracy of using this factor was confirmed by the SEM image analysis of the AuNP modified electrodes. A Cu/CuSO4 reference electrode was used instead of an Ag/AgCl to avoid chloride contamination since the presence of chloride ions will dissolve the deposited gold when high anodic potentials are applied to the electrode surface during the electrochemical AuNP surface analysis . The AuNPs at the GC electrode surface was electrochemically stripped off using a method described by Wang et al.  with minor alterations to adjust the conditions needed for enzyme dissolution. Here the AuNPs were stripped off into a 100-mM hydrochloric acid solution or in the case for enzyme dissolution the AuNPs were stripped off into a 100-mM acetate buffer solution containing 100 mM sodium chloride. To ensure all gold to be dissolved from the electrode surface, 6 sequential voltammetry cycles ranging from + 0.9 V to + 1.5 V (vs. NHE) with a scan rate of 0.1 Vs−1 was applied to the electrode surface, where generally all the gold was observed to be stripped off after the first cycle. The resulting oxidation peak observed at approximately + 1.3 V was integrated and together with the gold surface area as determined from the linear sweep analysis, was used for calculation of the average AuNP radius and the number of AuNPs electrodeposited at the electrode surface. After the AuNP stripping analysis, the sweep was repeated when electrodes were placed in a 500-mM H2SO4 solution, as described above, to ensure that all gold was eliminated from the electrode surface. To verify the accuracy in the electrochemical methods to determine AuNP size and surface coverage, SEM imaging of the AuNP coated electrode tip surfaces was performed using a LEO Ultra 55 FEG (Carl Zeiss, Germany) equipped with a field emission gun and an electron backscattered diffraction detector. The AuNP deposited electrodes were attached and grounded to an electrode holder using a tungsten wire for limiting the effect of charging during imaging. SEM image analysis of AuNP size and coverage was determined using the software Image J. (National Institutes of Health, USA).
Enzyme labeling with a fluorescent tag
For enzyme labeling with the fluorophore AlexaFluor488, a labeling kit protocol provided by Invitrogen (ThermoFisher Scientific) was used. Briefly, the enzyme, GOx, was first suspended in a phosphate-buffered saline (pH 7.4) at a 2 mg ml−1 concentration followed by raising the pH of the solution using sodium bicarbonate (pH 8.3). The enzyme solution was incubated with the fluorescent dye over night at 4 °C after an initial incubation of 1 h in room temperature with continuous stirring. The labeled enzyme was separated from excess dye using a size exclusion column according to the labeling protocol. To determine the average number of fluorophores attached per enzyme and the final enzyme concentration after the labeling procedure, the enzyme solution was analyzed using a Cary 4000 UV-Vis spectrophotometer (Agilent Technologies Inc., USA) according to the protocol provided by Invitrogen.
Immobilization of enzymes at the AuNP coated electrode surface
To immobilize enzymes at the AuNP coated surface of the GC electrode, the tip of each electrode was immersed into a 300 μL of a 10-mM sodium phosphate buffer solution containing fluorescently labeled GOx (0.2 mg mL−1), pH 7.4 for 3 h incubation time at room temperature. In this process enzymes attach through self-adsorption and after the enzyme coating process, the tip of each electrode was washed extensively with deionized water.
Quantification of the number of enzymes covering the AuNP surface of an electrode
The enzyme coated electrodes were immersed in to 200 μL of 100 mM acetate buffer containing 100 mM NaCl at pH 4. A chlorinated silver wire and a silver wire were used as reference and auxiliary electrode respectively in order to fit in the small volume used for the anodic stripping analysis. The chlorinated silver wire reference electrode was calibrated towards an Ag/AgCl saturated reference electrode prior to use. The AuNPs was electrochemically stripped off as described above, resulting in both dissolution of AuNP and freeing the AuNP adsorbed enzymes into solution. The electrochemical stripping solution containing the fluorescently labeled GOx was quantified using a Cary Eclipse fluorescence spectrophotometer (Agilent Technologies Inc., USA) and 494/519 nm as the excitation/emission wavelengths. A calibration curve for the labeled enzyme in acetate buffer (100 mM, pH 4) with 100 mM NaCl was performed for each new batch of labeled enzyme.
Results and discussion
In this work, we have developed a new fast, facile and non-toxic analytical method to characterize an enzyme-based AuNP coated biosensor with respect to the size and density of AuNPs at a carbon electrode surface together with quantification of the number of enzymes immobilized at the AuNP coating of the electrode. Since high curvature surface support as achieved with NPs has been shown beneficial for maintaining enzyme tertiary structure and thereby retaining enzyme activity upon immobilization, we here electrodeposited AuNP as enzyme support at the surface of a carbon electrode [12, 13]. We here characterize sensors for enzyme coverage and total electrode surface area. Sensors optimized for achieving monolayer coverage of enzyme adsorbed onto the AuNP coated electrode surface have shown to be favorable towards achieving both high sensor sensitivity and temporal resolution . Therefore, the coverage of the AuNPs at the electrode surface need to be optimized since too few NPs will lead to a poor enzyme loading and too many will lead to the formation of a gold film rather than discrete AuNPs, which results in losing the advantage with high curvature surface for retaining enzyme activity after immobilization. Using an electrochemical method for direct determination of the average AuNP size, the number of AuNP and the total AuNP surface area at the sensor surface replaces the need for scanning electron microscopy imaging analysis of each electrode for characterizing the nanostructure size and density at the surface, which is a time-consuming process that requires sampling of a number of representative images at the surface of each electrode for image analysis. The great advantage of the electrochemical approach is that the number of enzyme immobilized at the AuNP support at the sensor surfaces can be determined directly and related to each individual electrode. Due to the nature of electrochemical deposition of NPs at an electrode surface resulting in a variability in density and NP size at each individual electrode, this direct analysis method enables a direct characterization of each nanostructured electrode and facilitates determination of for instance enzyme loading at electrode NP surface coatings where NP size or density might vary.
Characterizing AuNPs size and coverage at the electrode surface
Counting the number of enzymes immobilized onto the AuNP surface
Comparison of AuNP size, total number of AuNP at the electrode surface, total AuNP surface area and surface coverage of electrodeposited AuNPs at the GC electrode surface when using three different conditions for the anodic stripping
AuNP diameter (nm)
Number of AuNP (109)
AuNP surface area (10−2) cm2
AuNP coverage %
19 ± 3
10 ± 4
5.2 ± 1.9
36 ± 13
17 ± 7
19 ± 11
6.5 ± 1.4
46 ± 10
Ac + GOx
15 ± 7
24 ± 11
4.3 ± 2.2
41 ± 10
Stripping was performed in HCl solution (HCl) (n = 25), acetate buffer (Ac) (n = 18) and in acetate buffer with enzymes immobilized at the surface (Ac + GOx) (n = 26). Results are presented as average values together with standard deviation.
GOx is an elongated globular protein consisting of two subunits with a dimension of approximately 8 nm along the major axis and 7 nm by the minor axis [39, 40]. As shown previously in literature, GOx have a tendency to flatten out at surfaces after adsorption depending on the surface properties of the immobilization support. This might also be affected by the concentration of enzyme at the surface, where crowding will hinder unfolding of the enzyme due to the lack of space to spread out [41, 42, 43]. Hence, even though the dimension of GOx is known, the number of enzyme that can be expected to fit onto a surface might deviate from theoretical calculation depending on the extent to which the tertiary structure is disrupted upon adsorption. At a surface where the unfolding of the protein structure is not initiated, the GOx footprint when adsorbing to a surface is estimated to range from 21 to 67 nm2 depending on if the enzyme attaches by the short or the long side to the solid support. However, at a flat gold surface the footprint was observed to increase up to approximately 290 nm2 when the enzyme totally collapsed onto the surface . Studies of the enzyme interaction with surfaces of other materials, with different topology and at different experimental conditions, shape changes of less extent were observed resulting in a footprints reported to vary in the range of about 70–144 nm2 when GOx adsorbed to these surfaces [41, 42].
To calculate the average number of enzymes attached per AuNP at the surface of each individual GC electrode (n = 6), the quantification of immobilized GOx at the AuNP surface of the electrode was directly related to the average AuNP size as determined electrochemically. Although the AuNP display a heterogeneous size distribution and a variability in particle density between individual electrodes, the population of AuNP at each individual electrode surface displayed a Gaussian distribution in terms of AuNP size and the resulting total AuNP surface area was rather constant. Hence, using the method for electrodeposition of AuNP offers roughly the same total available surface area for enzymes to bind and display an average diameter of AuNP corresponding to 14 ± 6 nm at the electrode surface.
As the interaction with a biomolecule to a AuNP surface is unique to the molecular properties of the biological component and is also related to the size of the AuNP, it is impossible to predict tendencies for macromolecular denaturation upon immobilization to a surface and therefore it is often a lot to gain in biosensor design and fabrication if studying the optimal conditions for the bioconjugation to ensure the bioactivity is ideal. In fabrication of enzyme based electrochemical sensors, the AuNP size affects sensitivity where for instance smaller AuNPs also possess increased electron density making them more electroactive and thereby more efficient in detecting the enzymatic product hydrogen peroxide. Hence, this electrochemical-based method offers a simple non-toxic way for quantitative analysis of fluorescently labeled immobilized biological component at the surface of biosensors based on gold bioconjugates. This analysis method therefore also should apply to many common biorecognition elements such as enzymes, antibodies, receptors, DNA or RNA bound to the sensor surface. In optimization of AuNP size used for immobilization of the biological component, this method can greatly facilitate this work and can also be used for quantification of co-immobilized biological components at a AuNP coated sensor surface. An example of this is for instance in finding the optimal conditions for immobilization of sequential enzymes that result in an optimum enzyme ratio to achieve prime sequential enzymatic reaction and can be performed by labeling each immobilized enzyme with an individual fluorescent tag.
Here we present a new facile, fast and non-toxic method to quantify the number of GOx enzyme that is immobilized to the surface of a AuNPs coating of a GC electrode. This method is based on combining an electrochemical analysis method, SEM image analysis and fluorimetry. An electrochemical method was used to characterize the density and size of electrodeposited AuNP at a GC electrode sensor surface and SEM image analysis was used to verify the electrochemical characterization of the AuNP structured sensor surface. After characterization of each individual AuNP coated GC electrode, these surfaces were coated with fluorescently labeled enzyme that attach through adsorption. Quantification of the immobilized enzyme was achieved by fluorescence measurement of the fluorophore-labeled enzymes after they were freed in solution by dissolving the AuNP support to which the enzymes were bound, using an electrochemical stripping technique. We show that by counting the enzymes bound to the AuNP surface we identified that the immobilization conditions used here result in roughly a monolayer coverage of enzymes at the electrode surface. And an enzyme footprint that indicate that depending on the level of enzyme using the available surface area to bind and spread upon adsorption, the enzyme at these conditions seem to be experiencing moderate to major shape changes. This analytical new method is of great importance for characterizing and optimizing the conditions needed to fabricate enzyme-based electrochemical biosensors, since an understanding of the surface processes for immobilizing the enzyme directly affects the sensor sensitivity and hence allows to determine the retained enzymatic activity after immobilization to the electrode surface. In addition, by limiting the enzyme coating to monolayer coverage is a key factor for further optimization of biosensors when it comes to achieving a high temporal resolution for detection of non-electroactive analytes by these biosensor probes. In summary, this method offers the possibility to characterize the sensor surface in terms of AuNP size and coverage and to perform quantitative analysis of different biological components that are used in bioconjugation when fabricating a biosensor. We show that fluorescent labeling allows for fluorescent quantitative analysis of the immobilized biomolecules, but depending on the properties of the biomolecules other direct quantitative analysis methods might be considered to determine the number of biomolecules bound to the surface before their support is electrochemically stripped off at the sensor surface. Therefore, we believe that this methodology might be very useful for optimizing the conditions when designing and fabricating many different kinds of biosensors where a biological recognition element is immobilized at an AuNP support.
The authors acknowledge the Swedish Research Council for funding. The authors would also like to thank Hoda Fathali for assisting with the SEM imaging of the AuNP coated electrodes and Hussein Mamkherzi for assisting with optimizing the conditions for deposition of AuNPs.
J.B., J.W. and A-S.C. designed the concept of this project. J.B designed all the experiments for this project and performed the experiments to functionalize the electrode surfaces with AuNPs, the experiments for electrochemical stripping to determine AuNP size and coverage at the electrode surface and the experiments to quantify number of enzymes covering the electrode surface using fluorimetry. Y.W. performed the SEM imaging of the AuNP coated electrodes and assisted with the SEM image analysis. J.B performed all the data analysis, interpreted the data and prepared all the figures for this manuscript. J.B. wrote the manuscript together with A-S.C. All authors discussed the results and commented on the manuscript.
Compliance with ethical standards
Conflict of interest
The authors declare that they have no conflict of interest.
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