Glucose Sensing Using Gold and Platinum Nanoparticle-Functionalized TiO2 Nanotubes

Biosensor research has concentrated on developing a simple sensing method with rapid detection, great sensitivity, and minimum interference. Comprising a photoelectric conversion and an electrochemical process, the photoelectrochemistry (PEC) process produces an extremely sensitive and precise signal with minimal background noise. 

Image Credit: Pixel-Shot/Shutterstock.com

PEC-based enzyme biosensors can solve poor selectivity issues in traditional bioanalysis techniques; the photoactive material not only acts as an energy converter but also provides the target receptors’ site in PEC to precisely find and acquire the target analyte.

In research published in the journal ACS Omega, TiO₂NTs/Au/Pt/GOx biosensor’s ultra-high sensitivity to glucose was achieved. The gold (Au) nanoparticles are loaded to improve the conductivity and increase the range of narrow light absorption of the material and the critical issues of electron–hole recombination.

Meanwhile, platinum (Pt) nanoparticles are deposited and employed as vital active components to catalyze the H₂O₂ decomposition under the enzymatic reaction. This leads to accelerated hydrogen peroxide consumption and the electrons transfer in the enzymatic reaction, thus obtaining ultra-high sensitivity for the PEC enzymatic glucose biosensor.

Results and Discussion

The PEC biosensor’s synthesis process diagram is illustrated in Scheme 1.

Scheme 1. Manufacturing Process of the TiO₂NTs/Au/Pt/GOx Biosensor. Source: Yang, et al., 2022

Depending on the scanning electron microscopy—SEM—image (Figure 1), the prepared TiO₂ nanotube film reveals a vertical one-dimensional tubular shape. The thickness and diameter of the nanotube are 20 and 80 nm, respectively. Because of the lower loading of Pt and Au nanoparticles, there was no considerable change observed on the surface of the electrode before and after the introduction of precious metals when compared with other electrodes.

Figure 1. (b) SEM images of TiO₂NTs/Au/Pt; (c,d) TEM and HR-TEM images of TiO₂NTs/Au/Pt; (e–i) HAADF-STEM and mapping of Ti, O, Au, and Pt of TiO₂NTs/Au/Pt; and (j) EDS spectra of TiO₂NTs/Au/Pt. Image Credit: Yang, et al., 2022

The researchers analyzed the structure of the electrode material, based on XRD tests on the electrodes. The Ti foil peak is indexed to the JCPDS database (44-1294), whereas the peaks situated on 25.28, 37.8, and 48.05° are designated to the crystal facets of anatase titanium dioxide (JCPDS 21-1272) at (101), (004), and (200), respectively, as shown in Figure 2a.

Figure 2. (a) XRD patterns of TiO₂NTs, TiO₂NTs/Pt, TiO₂NTs/Au, and TiO₂NTs/Au/Pt; (b) XPS spectra for Ti 2p; and (c) Au 4f core level for TiO₂NTs/Au/Pt. Image Credit: Yang, et al., 2022

In all the four systems (TiO₂NTs, TiO₂NTs/Pt, TiO₂NTs/Au, and TiO₂NTs/Au/Pt), UV–visible absorption spectra were measured (Figure 3a). An apparent absorption cut-off edge close to the anatase maximum absorption wavelength of 387 nm is shown by all four photoelectrodes This result proves that the noble metal load does not change the TiO₂ inherent absorption.

Figure 3. (a) UV–vis absorption spectra and (b) PL spectra of TiO₂NTs, TiO₂NTs/Pt, TiO₂NTs/Au, and TiO₂NTs/Au/Pt in the powder form at 350 nm excitation. Image Credit: Yang, et al., 2022

Depending on Figure 3b, the emission intensities were acquired. The pure TiO₂NTs have great PL intensity, highlighting the crucial recombination of photo-generated holes and electrons.

The PEC and electrochemical experiments were performed to analyze the PEC performance of the TiO₂NTs/Pt, TiO₂NTs, TiO₂NTs/Au, and TiO₂NTs/Au/Pt electrodes. The results of PEC reveal that the photocurrent density values are seen in the order: TiO₂NTs/Au/Pt > TiO₂NTs/Au > TiO₂NTs/Pt > TiO₂NTs. The findings confirm that precious metals are advantageous for photovoltaic performance.

Figure 4. (a) Photoelectric response of TiO₂NTs, TiO₂NTs/Pt, TiO₂NTs/Au, and TiO₂NTs/Au/Pt under simulated sunlight (bias voltage = 0.4 V, 0.1 M PBS, pH = 7.4); (b) electrochemical impedance spectra of Nyquist plots; (c) electronic lifetime measurement determined from the decay of the open-circuit potential in the absence of light; and (d, e) Mott–Schottky plot in the dark at a fixed frequency of 5 kHz. Image Credit: Yang, et al., 2022

The current density of TiO₂NTs and TiO₂NTs/Au electrodes in the absence of PtNPs were elevated, while one mM hydrogen peroxide is added to the buffer.

Assuming that Pt can decompose hydrogen peroxide in the solution and produce a large number of electrical signals under the potential applied for the catalytic oxidation, the densities between the TiO₂NTs/Pt and TiO₂NTs/Au/Pt electrodes altered with PtNPs that varied with 12.7 and 13 μA cm^–2 (vs Ag/AgCl), respectively, at 0.4 V. To improve electron transfer in enzymatic reactions, PtNPs were able to catalyze hydrogen peroxide.

Figure 5. CV comparison in 0.1 M pH = 7.4 PBS solution with and without a H₂O₂ chart of TiO₂NTs/Au/Pt. Image Credit: Yang, et al., 2022

The TiO₂NTs/Au/Pt/GOx electrode’s photoelectric response was tested at stepped glucose concentrations. The linear regression equation between glucose concentration and the photocurrent is illustrated in Figure 6.

Figure 6. (a) i–t responses of TiO₂NTs/Au/Pt toward Glu at concentrations of 0–4 mM in the supporting electrolyte of 0.1 M PBS (pH = 7.4); (b) linear calibration curve (glucose concentration vs photocurrent density); (c) stability after addition of 1 mM glucose under light conditions; and (d) anti-interference performance test performed in 0.1 M PBS (pH = 7.4) at concentrations of 0.1 mM chloride (NaCl), sucrose (Suc), ascorbic acid (AA), uric acid (UA), galactose (Gal), and fructose (Fru). Image Credit: Yang, et al., 2022

The researchers also showed the biosensor sensibility on the TiO₂NTs/Au/Pt/GOx electrode ten times after switching the i–t response.

According to Table 1, the TiO₂NTs/Au/Pt/GOx biosensor has better detection limits and higher sensitivity than the other PEC glucose biosensors.

Table 1. Comparison of the Analytical Performance of PEC Glucose Biosensors. Source: Yang, et al., 2022

The study describes the PEC detection mechanism of glucose at enzyme modification electrodes as depicted in Scheme 2.

Scheme 2. Mechanism of Action of the TiO₂NTs/Au/Pt/GOx PEC Biosensor for Glucose Detection. Source: Yang, et al., 2022

Methodology

Prior to the anodizing treatment, the pre-treated titanium foil sample of 10 × 20 × 0.25 mm size was polished for 30 seconds and further washed in ethanol, acetone, and deionized water for 5 minutes by ultrasonication and then dried at 50 °C in air. After which, in an electrochemical cell, anodic oxidation occurs with the working electrode being the titanium foil and the counter electrode being a stainless-steel sheet.

The electrolyte includes a glycol system comprising 6 vol % deionized water and 0.5 wt % ammonium fluoride. The specimen achieved by anodization was placed in a container and annealed in air at 400 °C for 3 hours with a two °C/min heating rate.

The precursor used was Chloroauric acid, the reference electrode used was Ag/AgCl, and the counter electrode used was Pt sheet. Gold nanoparticles were deposited electrochemically. Further, the prepared TiO₂NTs/Au was cleaned with deionized water and kept to dry in a vacuum oven for 12 hours at 70 °C.

By UV lamp deposition, nanoparticles of platinum were loaded on TiO₂NTs. On the target electrode, glucose oxidase is loaded by the drop coating method. Firstly, glucose oxidase was coated on the electrode surface and placed for 24 hours at four °C. Later, chitosan solution dissolved in acetic acid was provided dropwise to immobilize GOx and placed for 72 hours at four °C. Finally, TiO₂NTs/Au/Pt/GOx, the target biosensor, was obtained.

Conclusions

The study efficiently designed the high-performance PEC biosensor for glucose testing with TiO₂NTs/Au/Pt nanocomposites. Gold nanoparticles improved the electrode conductivity and promoted the light absorption through the SPR effect and the photogenerated carriers’ separation in the material by forming a Schottky junction. Consequently, the catalytic activity of the electrode was seen to improve greatly.

Simultaneously, PtNPs can catalyze the decomposition of H₂O₂ in the enzymatic reaction to improve the electrons transfer in the enzymatic reaction and further improve the PEC performance of the sensor. As a result, the achieved TiO₂NTs/Au/Pt/GOx PEC biosensor has great stability and selectivity.

Journal Reference

Yang, Z., Xu, W., Yan, B., Wu, B., Ma, J., W, X., Qiao, B., Tu, J., Pei, H., Chen, D., Wu, Q., (2022) Gold and Platinum Nanoparticle-Functionalized TiO₂ Nanotubes for Photoelectrochemical Glucose Sensing. ACS Omega, 7, 2, p 2474–2483. Available Online: https://pubs.acs.org/doi/10.1021/acsomega.1c06787.

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