Plasmonic Enhancement Coupling with Defect-Engineered TiO2–x: A

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Letter

Plasmonic enhancement coupling with defect-engineered TiO2x: a new mode for sensitive photoelectrochemical biosensing Jian Shu, Zhenli Qiu, Shuzhen Lv, Kangyao Zhang, and Dianping Tang Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.7b05296 • Publication Date (Web): 05 Feb 2018 Downloaded from http://pubs.acs.org on February 5, 2018

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Analytical Chemistry

Plasmonic enhancement coupling with defect-engineered TiO2-x: a new mode for sensitive photoelectrochemical biosensing Jian Shu, Zhenli Qiu, Shuzhen Lv, Kangyao Zhang, and Dianping Tang* Key Laboratory of Analytical Science for Food Safety and Biology (MOE & Fujian Province), State Key Laboratory of Photocatalysis on Energy and Environment, Department of Chemistry, Fuzhou University, Fuzhou 350116, People's Republic of China *Corresponding Author: Phone: +86-591-2286 6125. Fax: +86-591-2286 6135. E-mail: [email protected]. ABSTRACT: This work demonstrates that the photoelectric response of defect-engineered TiO2-x modified with Au nanoparticles can be modulated by oxygen vacancy concentration and excitation wavelength. Anchoring strongly plasmonic Au nanoparticles to defect-engineered TiO2-x by DNA hybridization, several times plasmonic enhancement of photocurrent occurs under 585 nm excitation, and it is employed as a novel signaling mode for developing an improved photoelectrochemical sensing platform. This signaling mode combining with exonuclease III-assisted target recycling amplification exhibits excellent analytical performance, which provides a novel photoelectrochemical detection protocol.

Photoelectrochemical (PEC) biosensing, an innovative sensing technique that integrates highly sensitive photochemistry with electrochemistry, has attracted huge attention because it usually exhibits superior detection performance in comparison with traditional methods.1-5 Moreover, this sensing system has been extensively expanded for different applications including environmental monitoring, food security analysis, clinical early diagnosis, and gene test. Despite some advances in this field, PEC biosensing modes are usually limited to the steric hindrance effect, the producing electron donors/acceptors, and energy transfer effect.4,5 So, developing new detection schemes based on novel signal-transduction principles is necessary because it substantially promotes PEC detection in a wide range of situations. Enormous achievements made in the fields of physical and material chemistry have provided a theoretical basis and reference for signaling model in PEC bioanalysis.6,7 A fruitful paradigm is plasmonic effect that recognized as an effective strategy for improving performance of solar cell and photocatalysts, 8-9 which has gradually employed to design advanced PEC biosensors.10-13 Immobilizing nano noble metals that show the collective oscillation of conduction electrons at visible light frequency onto semiconductors can affect the distribution of electromagnetic energy, which significantly alters the photocatalytic active and PEC response under visible light irradiation.14 This provides potential paths to design new PEC sensing platforms.15 For example, taking advantage of surface plasmon resonance (SPR) effect of Au nanoparticles (AuNPs), Dai et.al employing WS2/AuNPs nanocomposites developed a novel PEC biosensor for cell assay with red light excitation (630 nm).16 Our group designed a semiautomated PEC sensing platform based on decahedral BiVO4 that decorated with AuNPs on the high-active {010} facets.17 However, for the most SPR based PEC sensing system, these plasmonic metal nanostructures are modified to the semiconductors in advance and act as nanocomposites, and little attention has been paid to

the target-induced plasmonic metal nanostructures. Although these nanocomposites exhibit many special advantages such as enhanced absorption in the visible region, high photoconversion efficiency, and good biocompatibility, the pre-modified plasmonic metal nanostructures exhibiting SPR effect improve the background signal while they improve response signal in the almost reported strategies. Undoubtedly, the inherent disadvantage restricts the further improvement of analytical properties because they intimately depend on the characteristics of PEC active materials. Plenty of recent studies have shown that some important physicochemical properties of metal oxide semiconductors are closely related to intrinsic defects and extrinsic impurities.18,19 Oxygen vacancy, one of the most prevalent defects, can be introduced into wide bandgap semiconductors to efficiently expand optical absorption range and enhance the reactivity. Previous research has demonstrated that there was a synergistic effect between the oxygen vacancy and noble-metal deposition and it could be an effective strategy to improve the photocatalytic performance of TiO2.20 This inspires us that oxide defect-engineering may be a feasible strategy to develop advanced photoelectric materials for sensitive PEC biosensing. In our former research, we have also noticed that AuNPs could make an entirely different effect on photoelectric response of defect-engineered TiO2-x under different excitation wavelengths. Being mindful of these respects, the major goal of this work is to explore the plasmonic enhancement coupling with defectengineered TiO2-x nanobars (dTiO2-x) improving the visible light photoelectric response and subsequently apply it as a novel signaling mode for PEC biosensing with the ultralow background. Our experimental results indicated that the novel signaling mode combining with exonuclease III-assisted target recycling amplification exhibited excellent analytical performance for PEC biosensing. The PEC detection format described in this letter is based on a new mechanism and it can

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combine with some well-established approaches for further improvement.

Figure 1. (A) XPS spectra of O 1s for pristine TiO2 and dTiO2-x; (B) plotted d101 lattice spacing of dTiO2-x as a function of doping concentration of Fe3+ (inset: the corresponding XRD patterns); (C) Raman spectra of pristine TiO2 and dTiO2-x (inset: (left) the corresponding Raman peak of Eg vibrational mode and (right) relationship between peak positions of Eg mode and doping concentration of Fe3+); (D) UV-vis diffuse reflectance spectra (DRS) of pristine TiO2 and dTiO2-x (inset: band diagram of dTiO2-x).

dTiO2-x was employed as the photoactive species because defects affect the PEC properties significantly and play a crucial role in the PEC performance adjusting.21 Oxygen vacancies in the dTiO2-x were created by doping Fe3+ and its concentration could be conveniently controlled by the doping amount of Fe3+ based on the charge compensation (every two Fe3+ ions were compensated by an oxygen vacancy).22 Fe3+ doping has a negligible impact on morphology and size of TiO2 except that color gradually deepened to brown with the amount of Fe3+ increasing (Figure S1A-C). The XPS results (Figure S1D-F) indicated the incorporation of Fe into TiO2 lattice and Fe element existed in the form of Fe3+ because the peaks of Fe 2p 3/2 and Fe 2p 1/2 located at 710.7 and 724.2 eV, respectively. The negative shift of binding energy of Ti 2p observed in dTiO2-x also suggested the successful doping of Fe3+.23,24 Especially, the asymmetry of O1s XPS spectra located around 530 eV was changed obviously after incorporating Fe3+, which suggested the various chemical states of oxygen existing (Figure 1A).25 The peaks with the lowest (529.9 ± 0.2 eV) and highest (532.9 ± 0.2 eV) binding energy were ascribed to Ti-O bonds and surface adsorbed O-H bonds, respectively. The peak located at 531.0 ± 0.2 eV was attributed to the presence of oxygen vacancies in the host lattice.26-28 The relative peak areas increased from 34 % to 55 % after doping Fe3+ (0.5 %), which suggested Fe3+ doping effectively generating more oxygen vacancies. No impurity phase was observed by XRD analysis and all diffraction peaks of dTiO2-x samples with different amount of Fe3+ were matched well with anatase phase (Figure 1B). The gradual shift of anatase (101) peak to smaller d101 lattice spacing with the increase of Fe3+ concentration indicated more oxygen vacancies were created as charge-compensating defects for incorporated Fe3+ in TiO2 lattice.22 Because the increase of oxygen vacancies within the dTiO2-x can result in Raman peak (Eg mode at around 143 cm-1) shifting towards lower wavenumber,29 the Eg mode shifting with the increase of Fe3+ concentration in dTiO2-x was observed, which directly demon-

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strated the positive correlation between the concentration of oxygen vacancies and doped Fe3+ (Figure 1C). The effect of generated oxygen vacancies on the spectral properties of dTiO2-x was revealed by UV−Vis−DRS spectra (Figure 1d). Compared with pristine TiO2, the red shift in absorption edge and absorption intensification in visible range were observed in dTiO2-x, which could be ascribed to the oxygen vacancies narrowing the band gap of TiO2 (Figure 1d, inset).30 The appropriate oxygen vacancies not only enhances the light absorption but provides reactive sites to improve the PEC activity of dTiO2-x. Here, to explore the effect of AuNPs on the photoelectric response, the transient photocurrent of dTiO2-x was recorded. Expectedly, there were obvious differences between the photocurrent intensities before (Ib) and after decorating AuNPs (Ia). The differences were reliant heavily on the excitation wavelength (Figure 2A). Under UV illumination, the existence of AuNPs dampened photocurrent by 25 % because it hindered the photon flux of shortwave reaching the dTiO2-x and reduced the interface between dTiO2-x and electrolyte. However, the addition of AuNPs resulted in a 1-fold increase of photocurrent under visible light illumination. Specifically, we investigated the effects of particular wavelength on the increasing rate R (R=Ia/Ib) (Figure S2). As diagrammed in Figure 2B, different excitation wavelengths had significantly different effects on R and 585 nm generated maximal R (6.7) under the same condition. The low photoelectric response of dTiO2-x under visible light illumination originated from the oxygen vacancies.31,32 Thus, oxygen vacancies should play an important role in photocurrent increase, which was confirmed subsequently (Figure 2B, inset). Stimulated by the light of 585 nm, the R for pristine TiO2 was only 2.4 and maximal R was achieved at 0.5 % doping Fe3+. The plasmonic enhancement of photocurrent provided by the AuNPs is well matched to this defect-rich matrix. It's worth mentioning that the significant increase in photocurrent of the AuNPs-dTiO2-x system under 585 nm light illumination was based on the ultralow background. Though the doping Fe3+ extended the light absorbance range and provided more active sites, PEC efficiency decreased with over doping Fe3+ because the excess oxygen vacancies provided more recombination site for charge carriers. On the other hand, the photocurrent increase was more obvious under irradiation of light that far below the band gap of dTiO2-x (2.5 eV), thus, an extension of the absorbance range by over doping Fe3+ did little to photocurrent increase. Introduction of AuNPs significantly enhancing the photocurrent could be ascribed to hot electron transfer according to the excitation wavelength.14,33 One of the most important characteristics of AuNPs is the strong SPR effect and the surface plasmon decay generates electron-hole pairs within the AuNPs. The generated hot electrons with higher negative potential than that of the conduction band (CB) of dTiO2-x could inject smoothly into CB, which resulted in enhancement of photocurrent (Figure 2C).34 In this charge transfer mechanism, the AuNPs acting as a sensitizer to improve the photocurrent was analogous to the dye in dye-sensitized solar cell. Moreover, the photocurrent also increased slightly in a wavelength range that the plasmon resonance and semiconductor resonance overlapped (450-520 nm), which may contribute in part by plasmon resonance energy transfer (PRET) and resonant light scattering effect of AuNPs.15 Under the light irradiation, the PRET enhanced the local electric field intensity in a small region around the dTiO2-x, which not only improved the light

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Analytical Chemistry absorption rate of dTiO2-x by capturing more photos from the area larger than its geometric cross section but (Figure 2D) prompted the electron-hole pair generating rate of dTiO2-x.35,36 The resonant light scattering effect may also improve the photoabsorption by increasing the path length of photo in the semiconductor films.37

Figure 2. (A) Photocurrent of dTiO2-x (a) in the absence and (b) presence of AuNPs under UV and Vis light irradiation (inset: magnification of photocurrent curve under Vis light irradiation); (B) the effect of excitation wavelength and doping concentration of Fe3+ (inset) on the photocurrent increase rate R; schematic illustration plasmonic photosensitization effect: (C) hot electron transfer and (D) PRET enhanced the electric field.

The plasmonic enhancement coupling with dTiO2-x enhancement mechanism may provide a novel signaling mode for developing an improved photoelectrochemical sensing platform. To verify the feasibility of this signaling mode, a well-developed biosensor was employed as depicted in Figure 3A. The amino-terminated capture DNA (cDNA) was immobilized to the electrode surface facilely conjugating to carboxylic groups to form PEC biosensing interface. At the same time, an exonuclease III (Exo III)-assisted target recycling amplification strategy was designed ingeniously to generate more AuNP-tagged rDNA segments that anchor the AuNPs to the electrode by specifically hybridizing with cDNA (Scheme S2). Firstly, the feasibility of Exo III-assisted signal amplification was demonstrated by online software suite (Figure S3) and polyacrylamide gel electrophoresis (Figure S4A). Additionally, the signal amplification effect of Exo III was further proved by the verified experiments (Figure S4B). To highlight the advantage of plasmonic enhancement coupling with dTiO2x, a similar PEC biosensing interfaces was constructed by using pristine TiO2 as a control (Figure 3B). Under light (585 nm) irradiation, the two biosensors exhibited background signal with similar intensity (Figure 3C). Upon addition of tDNA (5 nM), only a minor increase in photocurrent intensity exhibited by control. However, a pronounced increase was observed in dTiO2-x-based biosensor under the optimized experimental conditions (Figure S5). The anchored AuNPs provided plasmonic effect and increased photocurrent of the photoactive matrix. On the one hand, this result indicated that the more efficient enhancement effect between plasmonic photosensitization of AuNPs and defect-rich in dTiO2-x, which greatly improved the PEC sensitivity. On the other hand, the increase of photocurrent directly related to the tDNA amount, such a significant positive correlation could be used to quantify the tDNA concentration. As shown in Figure 3D, the photocurrent

varied linearly with the logarithm of synthetic tDNA concentration in the range of 1.0 pM - 10 nM. The linear fitting function between logarithm of tDNA concentration and variation of photocurrent (∆I = Itarget - Ibackground, where Itarget and Ibackground are the stable photocurrent in the presence and absence of target) was ∆I (nA) = 3.616 + 14.450 × logCtDNA (pM, R2 = 0.967, n = 5) with a detection limit of 0.6 pM (S/N = 3).

Figure 3. Schematic illustration of PEC biosensor constructed by using (A) dTiO2-x and (B) pristine TiO2; (C) Photocurrent responses of dTiO2-x and pristine TiO2-based PEC biosensor toward 0 and 5 nM tDNA; (D) The linear correlation between ∆I and logarithm of the tDNA concentrations (inset: the corresponding photocurrent toward tDNA); (E) the photocurrents toward 0.5 nM of (a) single-base mismatched DNA, (b) two-base mismatched DNA, (c) tDNA, (d) noncomplementary DNA and (e) blank sample (Error bars represent the standard deviation of three parallel detection).

The photocurrent toward different DNA sequences including target DNA, single-base mismatched, two-base mismatched and non-complementary DNA were measured to evaluate the specificity of the biosensor. Since single-base mismatched and two-base-mismatched DNA could open the pDNA inefficiently, they also generated PEC signal on a certain level (Figure 3E). The signal of non-complementary strand paralleled to the background. Though these DNA sequences exhibited different responses, the response to tDNA was significantly greater than those of others, demonstrating the satisfactory specificity. Moreover, results also showed that the stability, reproducibility, and precision were acceptable by comparing the relative standard deviation (Supporting Information). These results powerfully evidenced that the plasmonic enhancement of AuNP to dTiO2-x for PEC signaling by combining with Exo III-assisted target recycling amplification was a robust detecting method. In summary, this work explored the plasmonic effect of Au NP on dTiO2-x depending on defect concentration and excitation wavelength. Employing plasmonic effect as a novel signaling mode, a target-induced plasmonic enhancement PEC biosensor for sensitive DNA detection with Exo III-assisted target recycling amplification was proposed. The dTiO2-x used as a photoactive matrix expanded absorbance to the visible light and exhibited low background. The introduced AuNPs generating hot electrons inject into the TiO2-x and also enhancing the local electric field intensity improve the light absorption rate of dTiO2-x, which significantly multiply the photocurrent under visible light irradiation. The powerful integration of

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this PEC signaling mode with highly efficient Exo III-assisted target recycling amplification made it sensitive to targets. The signaling mode provides a novel PEC detection format, which enjoys bright prospect and may score more fruitful achievements in the area of bioanalysis.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.analchem.0000. Experimental details, TEM images, photographs and XPS of nanomaterial, effects of excitation wavelength on photocurrent, evaluation of the feasibility of signal amplification, optimizing concentrations of pDNA and Exo III, additional evaluation of analytical performance (PDF).

AUTHOR INFORMATION Corresponding Author * Phone: +86-591-2286 6125. Fax: +86-591-2286 6135. E-mail: [email protected] (D. Tang).

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was financially supported by the National Natural Science Foundation of China (21675029 & 21475025), and the Program for Changjiang Scholars and Innovative Research Team in University (IRT15R11).

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Analytical Chemistry

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