ORIGINAL PAPER

Photoelectrochemical determination of Hg(II) via dual signalamplification involving SPR enhancement and a folding-basedDNA probe

Yushu Shi1 & Guoqing Zhang1 & Jiaojiao Li1 & Yong Zhang1 & Yanbao Yu2& Qin Wei1

Received: 13 October 2016 /Accepted: 17 February 2017 /Published online: 24 February 2017# Springer-Verlag Wien 2017

Abstract The authors describe a highly sensitive and se-lective photoelectrochemical (PEC) assay for mercury(II)ions. It is based on a dual signal amplification strategy.The first enhancement results from the surface plasmonresonance (SPR) of Au@Ag nanoparticles (NPs)absorbed on MoS2 nanosheets. Here, the injection ofhot electrons of Au@Ag NPs into MoS2 nanosheets pro-duces a strong photocurrent, while background signalsare strongly reduced. The second enhancement resultsfrom the use of a thymine rich ct-DNA aptamer attachedto the Au@Ag-MoS2 nanohybrid. The DNA specificallybinds Hg(II) ions to form thymine-Hg(II)-thymine (T-Hg-T) complexes. This leads to the formation of a hairpin-shaped dsDNA structure. The use of a CdSe quantum dotlabel at the terminal end of the ct-DNA further facilitateselectron–hole separation. The photocurrent of the detec-tor is measured as a function of Hg(II) concentration at abias voltage of 0.1 V and under irradiation of 430 nmlight. Due to the two-fold amplification strategy present-ed here, the linear range extends from 10 pmol·L−1 to100 nmol·L−1, with a detection limit of 5 pmol·L−1 (at S/N = 3).

Keywords Au@Ag .MoS2 . T-Hg-T . Nanosheets . CdSequantum dots . Nanohybrid . Au@Ag-MoS2 . Indium tinoxide . HRTEM . Electrochemical impedance spectroscopy

Introduction

Mercury(II) ion (Hg2+) is one of the most toxic and dan-gerous heavy metal pollutants in environment, and it hasbeen listed as a prior pollutant by many countries andinternational agencies [1, 2]. The U.S. EnvironmentalProtection Agency (EPA) and the international WorldHealth Organization (WHO) specify maximum allowablelevels of Hg2+ as 2 and 6 ppb in drinking water, respec-tively [3].

Many attempts have been made to develop varioussensors for rapid detecting Hg2+ [4–9]. For example,Zarlaida and Adlim detailed reviewed colorimetricmercury(II) ions detection using spot test, paper andglass strips containing the active agent made of Au/Agmetal nanoparticles and dyes [10–12]. Among these tech-niques, the photoelectrochemical (PEC) assay attractsspecial interest [13–15]. Coupling photo irradiation withelectrochemical detection, the PEC technique possessespotentially higher sensitivity than the conventional elec-trochemical methods, due to the reduced background sig-nals obtained from the separation of the excitation sourceand the detection signals [16, 17]. To realize the sensi-tive and selective detection of Hg by means of PECmethod, different modified materials and different sens-ing strategies have been studied [18]. For example, Liet al. reported that due to the energetic electrons fromthe surface plasmons of the nanogold were injected intothe LUMO orbit of the organic perylene-3,4,9,10-tetracarboxylic acid semiconductor and then rapidly

Electronic supplementary material The online version of this article(doi:10.1007/s00604-017-2141-3) contains supplementary material,which is available to authorized users.

* Yong Zhangyongzhang7805@126.com

1 Key Laboratory of Chemical Sensing & Analysis in Universities ofShandong, School of Chemistry and Chemical Engineering,University of Jinan, Jinan 250022, People’s Republic of China

2 J. Craig Venter Institute, 9714 Medical Center Drive,Rockville, MD 20850, USA

Microchim Acta (2017) 184:1379–1387DOI 10.1007/s00604-017-2141-3

transferred to the electrode, Hg2+ was detected sensitive-ly under the visible light (λ > 450 nm). By means of CdSQD-tagged mercury-specific oligonucleotides, Ma et al.reported a thymine-Hg2+-thymine (T-Hg2+-T) folding-based PEC sensor for Hg2+ detection.

Due to the 2D ultrathin atomic layer structure and highsurface area, MoS2 nanosheets (NSs) showed uniquephysical and electrical properties, which made it very in-teresting in the fields of energy storage and biosensor [19,20]. Especially, the MoS2 NSs have potential applicationsin PEC assay because their property of decreased chargetransfer resistance can dramatically promote the PEC per-formance. However, pristine MoS2 NSs have poor elec-tron transport property and then always require very largeover potential in PEC operation to achieve considerableturnover frequency [21]. Au, Ag and Au@Ag core@shellnanoparticles (NPs) have been regarded as a versatile tem-plate for the immobilization of biomolecules and also playimportant roles in numerous fields of biomedical applica-tions, because of their merits of easy preparation, highhomogeneity, and biocompatibility, robust and stable, es-pecially surface plasmon resonance (SPR) [22, 23]. BySPR excitation of Au@Ag nanostructure, hot electronscan be injected from Au@Ag NPs into MoS2 NSs [24].This phenomenon results in an increase of charge densityof MoS2 NSs and, in turn, modulation of the energy levelof the sensor more comparable to PEC. Thus, SPR en-hanced photoelectric activity for PEC can be achieved.

Herein we proposed a plasmon-activated PEC system ofAu@Ag NPs/MoS2 NSs (Au@Ag-MoS2) nanohybrid, inwhich Au@Ag NPs acting as a light absorber excites elec-tron–hole pair during SPR process and MoS2 NSs acting asactive sites and electron acceptor facilitate PEC. For doubleenhancing the PEC signals to improve the analysis sensitivity,water soluble CdSe quantum dots (QDs), which have beenemerged as a significant class of PEC materials in analyticalchemistry over the past decade [25, 26], are introduced in thisassay. By means of the thymine - thymine (T-T) pair, whichcan selectively capture Hg2+ [27], this PEC assay can be usedfor detection of Hg2+ in real sample.

Experimental section

Materials and reagents

Indium-tin-oxide (ITO) glasses (surface resistivity ≤10 Ω·m−2) were purchased from Zhuhai Kaivo OptoelectronicTechnology Electronic Components Co., Ltd. (http://www.zh-kv.com). All chemicals were used without furtherprocessing. HAuCl4·3H2O and AgNO3 were obtained fromShanghai Chemical Reagent Co., Ltd. (http://ccn.mofcom.gov.cn) and used as received. Cetyltrimethylammonium

bromide (CTAB), ascorbic acid (AA) and other reagents werepurchased from Nanjing Chemical Reagent Co., Ltd. (http://www.njhuaxuesj.cn/). Phosphate buffer (0.1 mol·L−1, pH 7.4)(0.1 mol·L−1 Na2HPO4 mixed with 0.1 mol·L−1 KH2PO4) wasused as electrolyte through the experiment. MoS2 powder andn-butyllithium in hexane (1.6 mol·L−1) were purchased fromSigma-Aldrich (http://www.sigmaaldrich.com/). Thioglycolicacid (TGA), N-hydroxysuccinimide (NHS) and 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide hydrochloride (EDC)were obtained from Tianjin Kermel Chemical Reagent Co.,Ltd. (http://www.chemreagent.com/). EDC/NHS solution iscontaining 0.01 mol·L−1 of EDC and 0.002 mol·L−1 of NHSin 4 mL ultrapure water. Ultrapure water was prepared by aMillipore Milli-Q system and used throughout.

Apparatus

Photocurrent was measured on a PEC workstation(ZahnerZennium PP211, Germany) at a bias voltage of0.1 Vwith light intensity of 150W·m−2. The distance betweenthe light source and the electrode was fixed at 10 cm.Electrochemical impedance spectroscopy (EIS) was per-formed on an autolab impedance measurement unit (IM6e,Zahner Elektrik, Germany) in 5.0 mmol·L−1 [Fe(CN)6]

3-/4-

solution containing 0.10 mol·L−1 potassium chloride (KCl),and recorded with an amplitude of 5 mV over a frequencyrange of 0.1 Hz to 100 kHz. The electrochemical apparatusis a conventional three electrodes system with modified ITOelectrode as the working electrode, Pt wire electrode as theauxiliary electrode and saturated calomel electrode as the ref-erence. The UV–vis absorption spectra are recorded onShimadzu UV3600 UV–vis-NIR spectrophotometer(Lumerical Solutions, Inc.). XRD patterns of the preparedsamples were acquired with a Rigaku D/MAX 2200 X-raydiffractometer (Tokyo, Japan, Braggequation 2d sin θ = nλ,n = 1, λ = 0.154 nm). Transmission electron microscopy(TEM, JEOL JEM 1200EX working at 100 kV) and high-resolution TEM (HRTEM, FEI Tecnai G2 F20 S-Twin work-ing at 200 kV) were utilized to characterize morphology andinterfacial lattice details. Scanning electron microscope(SEM) images were obtained using a field emission SEM(Zeiss, Germany).

Preparation of Au NPs and Au@Ag NPs

Gold NPs and Au@Ag NPs were prepared via the previouslyreported method of seed-mediated growth with little modifi-cation [28]. Briefly, Au-seeds were first made by adding0.6 mL of ice-cold NaBH4 solution (10 mmol) into 10 mLaqueous solution containing HAuCl4 (0.25 mmol) and CTAB(5 mmol), and then Au NPs were prepared by successivelyadding 3.8 mL AA solution (0.38 mmol) and two drops of Auseeds into 40 mL aqueous solution in the presence of HAuCl4

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(8 mmol) and CTAB (64 mmol). After centrifugation at6500 rpm (equivalent to a relative centrifugational force of2810) for 15 min, the products re-dispersed into 3 mLCTAB solution (3 mmol). Au@Ag NPs were synthesized bysuccessively adding 0.3 mL of AA (3 mmol) aqueous solu-tion, 0.1 mL of AgNO3 (0.001 mmol) aqueous solution and0.3 mL of NaOH (3 mmol) aqueous solution. After 2 h, theAu@Ag NPs colloid was collected by centrifugation andwashed with deionized water twice. Following the last centri-fugation, the products re-dispersed into 3 mL CTAB solution(3 mmol) to produce the Au@Ag NPs colloid.

Preparation of MoS2 NSs and Au@Ag-MoS2 nanohybrid

TheMoS2 nanosheets were synthesized according to literatureprocedure [29]. Briefly, pristine bulk MoS2 was intercalatedwith lithium by reacting MoS2 powder (0.6 g) with n-butyllithium in hexane (1.6 mol·L−1, 6 mL) at 60 °C under argonatmosphere. After 2 days, the suspension was filtered over a450 μm pore size membrane (Millipore) and washed with60 mL of hexane for three times, giving a black powder ofintercalated MoS2 compound. Exfoliation was then immedi-ately suspended in 30 mL ofMillipore water and sonicated for1 h. Exfoliated material was dialyzed for 5 days. The solutionwas centrifuged for several times to remove the un-exfoliatedmaterials. About 50 mL of supernatant was collected and usedimmediately.

The Au@Ag-MoS2 nanohybrid was prepared by mixingboth the Au@Ag NPs and MoS2 NSs solutions with a volumeratio of 1:1. Then, the mixture was sonicated for more than 1 h.

Preparation of water soluble CdSe QDs

The water soluble CdSe QDs were synthesized using the re-ported method [25]. Briefly, dissolving 10 mmol Se powderand 12 mmol Na2SO3 in 50 mL ultrapure water under mag-netic stirring at 90 °C for 3 h will get the Na2SeSO3 solution.Then, 0.84 mL TGA diluted with 60 mL distilled water wasadded into 40 mL of 0.125 mol·L−1 CdCl2·2.5 H2O solutionunder magnetic stirring. Thereafter, the pH value of the solu-tion was adjusted to 10 with 1.0 mol·L−1 NaOH and 12 mL ofthe Na2SeSO3 solution was added. The solution was heated toboiling and refluxed for 4 h. Finally, TGA-stabilized CdSeQDs were prepared.

Detection procedure of the PEC assay

The performance of the PEC assay was tested by itsphotocurrent-time response in pH 7.4 of 0.1 mol·L−1 phos-phate buffer containing 0.20 mol·L−1 of AA, which servedas a sacrificial electron donor during the photocurrent mea-surement [30]. Photocurrent was measured by the current-timecurve experimental technique on a PEC workstation at a bias

voltage of 0.1 V and under irradiation of 430 nm light. ForHg2+ detection, the original photocurrent signal (I0) of PECassay was firstly measured at the above-mentioned condition.After adding Hg2+, the residual photocurrent signal (I1) wasrecorded at the same condition. The photocurrent increasefactor (ΔI) caused by Hg2+ was calculated as follows: ΔI(μA) = I1 - I0.

Results and discussion

Characterization of morphology and structure

The morphology of the MoS2 nanosheets was characterizedthrough SEM. As shown in Fig. 1a, the shape of MoS2 ismultilayer and thin wrinkled paper-like structure, which canprovide large surface for loading more photoactive materialsand the suspension was homogeneously dispersed. These re-sults convinced that the chemically exfoliated method was typ-ically considered as an efficient pathway to produce active lay-ered MoS2 NSs in high yield. In addition, the prepared MoS2NSs were also confirmed by XRD patterns in Fig. 1b. Intensediffraction peaks at 2θ = 14.1°, 32.8°, 39.6°, 48.9° and 58.4° arecorresponding to (002), (100), (103), (105) and (110) latticeplanes respectively. These XRD pattern results match well withthe hexagonal phase of MoS2 (JCPDS 37–1492) [31].

The TEM images of Au@Ag NPs in Fig. 1c and d show alarge number of Au@Ag NPs with uniform morphology ofthe average diameter ~35 nm, indicating the monodisperseAu@Ag NPs. Figure 1d shows the SEM photograph ofAu@Ag-MoS2 nanohybrid colloidal sol and the Au@AgNPs are well dispersed and absorbed on the surface of MoS2NSs. Corresponding UV–vis spectra is shown in Fig. 1f. Theprepared Au@Ag NPs (curve b) show a local SPR peak at410 nm, which is blue shift of ~115 nm compared with AuNPs (curve a). The absorption spectrum implies that theAu@Ag NPs have a broad absorption range and is suitablefor employed as photoactive substrate. Surprisingly, Au@Ag-MoS2 nanohybrid (curve c) shows an absorption peak at430 nm compared with pure MoS2 NSs (curve d), which hasno absorption peak in this field, indicating successfully syn-thesized of Au@Ag-MoS2 nanohybrid as composition ofthese two kinds of materials and also indicating SPR enhancedphoto absorption at 430 nm.

The TEM image of CdSe QDs in Fig. 1g reveals the aver-age diameter is ~3.5 nm. Figure 1h depicts the UV–vis ab-sorption and PL emission spectra of the TGA-stabilized CdSeQDs. The characteristic absorption spectrum implies that theCdSe QDs have a broad absorption range at around 465 nm(curve a), which allows for the efficient excitation, and goodsymmetry and relatively narrow spectral width of emissionspectrum (curve b), which provides sufficient spectral

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resolution of the fluorescence intensity. So, CdSe QDs aresuitable for employed as photoactive materials in this study.

Characterization of the procedure of preparing PEC assay

Figure 2 illustrates the schematic diagram of the stepwise ofthe folding-based Hg2+ assay fabricating process and PECanalysis of Hg2+ though the T-Hg2+-T complex. Firstly,Au@Ag-MoS2 nanohybrid is prepared through a syntheticprocedure (Fig. 2a). Secondly, as shown in Fig. 2b, for prep-aration of PEC assay, 6 μL of the homogeneous suspension ofthe Au@Ag-MoS2 colloidal sol is firstly dropped onto a pieceof ITO electrode. After drying in air, 6 μL of 2 mg·mL−1 ct-DNA solution is dropped onto the ITO/Au@Ag-MoS2 elec-trode and dried at room temperature. Then, 6 μL of the EDC-NHS is dropped on electrode for 1 h at room temperature,followed by thoroughly rinsing with washing buffer to washoff the excess EDC and NHS. At the same time, 6 μL of TGA-stabilized CdSe QDs solution is dropped on EDC/NHS acti-vated ITO/Au@Ag-MoS2/ct-DNA electrode. After the wash-ing step, the PEC assay is prepared, namely ITO/Au@Ag-MoS2/ct-DNA/CdSe.

Electrochemical impedance spectroscopy (EIS) is an effec-tive approach for characterizing the interface properties ofelectrodes and demonstrating the full modification processof sensors [32, 33]. As shown in Fig. 3a, an almost straightline with a very small semicircle for the EIS of the bare ITOelectrode exhibits in the impedance spectrum indicating a verysmall Ret value (curve a), which can be characteristic of a

diffusional limiting step of the electrochemical process.Compared with bare ITO electrode, the semicircle diametersignificantly increases after the electrode has been modifiedon the surface of ITO electrode with Au@Ag-MoS2 (curve b),which indicates that Au@Ag-MoS2 film has been successful-ly anchored on the surface of ITO electrode and inhibitedelectron transfer from the electrode to the solution in the sys-tem. After ct-DNA has been modified on the Au@Ag-MoS2film, there is a little increased electron transfer resistance(curve c), suggesting ct-DNA has been conjugated onAu@Ag-MoS2 functionalized electrode. Subsequently, theRet value gradually increases after the EDC/NHS and CdSeQDs has been modified on the ITO/Au@Ag-MoS2/ct-DNAelectrode surface in turns (curve d and curve e). Herein, EDC/NHS is used for linking the -COOH groups of CdSe QDs and-NH2 groups of ct-DNA. So, the increased resistance on thesurface of the modified electrode indicates the successful as-sembling of them.

To further demonstrate the full modification and detectionprocess of the assay, PEC behavior is used to characterize theinterface properties of electrodes (Fig. 3b). There is no photo-current monitored on bare ITO electrode (curve a) with thelack of photoelectric active materials. As expected, if ITOelectrode is only modified with MoS2 NSs, no obvious pho-tocurrent also emerges and the spectrogram coincides withcurve a. Curve e presents the photocurrent response ofAu@Ag-MoS2 modified ITO electrode. The photocurrent in-tensity of Au@Ag-MoS2 increases extremely higher than thatof MoS2 NSs modified electrode, which indicates plasmon

Fig. 1 (a) SEM image and (b) XRD pattern of MoS2 NSs; (c) lowresolution and (d) high resolution TEM images of Au@Ag NPs; (e)SEM image of Au@Ag-MoS2 nanohybrid; (f) UV–vis spectra of (a)

Au NPs, (b) Au@Ag NPs, (c) Au@Ag-MoS2 nanohybrid, (d) MoS2NSs; (g) high resolution TEM image of CdSe QDs; (h) (a) UV–visspectra and (b) PL spectra of CdSe QDs

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enhanced MoS2 can be well used as photoelectric active ma-terials tomodified the PEC electrode. After the ct-DNA (curved), EDC/NHS (curve c) and CdSe QDs (curve b) have been

modified on the Au@Ag-MoS2 film surface in turns, the pho-tocurrent intensity gradually decrease, owing to the increasedresistance on the surface of the electrode.

Fig. 2 a Preparation procedure of Au@Ag-MoS2 nanohybrid. b Preparation and detection procedure of PEC assay

Fig. 3 a EIS characterization of the sensing interface: (a) bare ITO, (b)ITO/Au@Ag-MoS2, (c) ITO/Au@Ag -MoS2/ct-DNA, (d) ITO/Au@Ag-MoS2/ct-DNA / EDC/NHS, (e) ITO/Au@Ag -MoS2/ct-DNA / EDC/NHS/CdSe QDs. b Photocurrent response of the modified electrode (a)

bare ITO and the same as ITO/MoS2, (b) ITO/Au@Ag -MoS2/ct-DNA /EDC/NHS/CdSe QDs, (c) ITO/Au@Ag -MoS2/ct-DNA / EDC/NHS, (d)ITO/Au@Ag -MoS2/ct-DNA, (e) ITO/Au@Ag-MoS2, (f-h) after differ-ent concentration of Hg2+ solution (1, 10, 100 nmol·L−1) incubation

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When the constructed assay was tested, the photocurrentdisplays an obvious increase while adding Hg2+ into PECsystem (curve f to h). With Hg2+ concentration increased, agreater proportion of the folding configuration can be gottenand higher electron transfer efficiency from CdSe QDs toAu@Ag-MoS2. As a consequence, rising photocurrent canbe monitored. Based on this proportional amplification effect,highly sensitive detection for Hg2+ is accomplishedsuccessfully.

Mechanism of the PEC assay

As shown in Fig. 3b, there is negligible photocurrent responseafter the electrode has beenmodified only withMoS2 NSs, butthe Au@Ag-MoS2 nanohybrid exhibits increased light ab-sorption intensity at the wavelength of 430 nm. These resultsreveal two important features. One is that the negligible photoresponse of pure MoS2 NSs in the visible region explains thesmall photocurrent obtained under visible-light illumination(curve d in Fig. 1f). The other is that Au@Ag-MoS2nanohybrid shows enhanced photo activity at 430 nm, whichis well-matched with its corresponding SPR absorption peaks(curve c in Fig. 1f). These results well demonstrate that SPReffect can effectively enhance the UV–vis absorption perfor-mance of the MoS2 NSs. Moreover, in the presence of Hg2+

ions, the ct-DNA aptamer molecules with CdSe QDs can formT-Hg2+-T structure and lay down on the surface of theAu@Ag-MoS2 in a flexible manner. When the detection

proceeded, CdSe QDs exit via a corrosion process under illu-mination: 2 h+ + CdSe→Cd2+ + Se, in which AA is used as anelectron donor to suppress the recombination of electrons andholes of CdSe sensitized electrode. Therefore, as shown inFig. 3b, the modified electrode can obtain enhanced and stablephotocurrent intensity. Furthermore, the dual signal amplifica-tion affects the sensing performance significantly (curve f-h inFig. 3b).

Optimization of experimental conditions

To obtain the best performance for PEC detection of Hg2+, (a)the effects of pH, (b) the irradiation wavelength of light, (c)AA concentration and (d) the volume ratio of Au@Ag NPsand MoS2 NSs mixed in preparation of Au@Ag-MoS2nanohybrid solution were investigated on the photocurrentresponse of assay. Respective data and figures are given inthe Electronic Supplementary Material (ESM). The followingexperimental conditions were found to give best results: (a) Asample pH value of 7.4, (b) the irradiation wavelength of430 nm, (c) AA concentration of 2.0 mol∙L−1, (d) the volumeratio of Au@Ag NPs of 1:1.

Performance of the PEC assay

Figure 4a displays the resulting photocurrents after addingHg2+ with different concentrations and the corresponding de-rived calibration curve is shown in Fig. 4b. Under the

Fig. 4 a Photocurrent responseof the aptasensor at differentconcentrations of Hg2+: 0.01 (a),0.05 (b), 0.1 (c), 0.5 (d), 1.0 (e),5.0 (f), 10 (g), 50 (h) and100 nmol·L−1 (i); b Calibrationcurve of Hg2+ detection. Inset:Plot of photocurrent change vsHg2+ concentration; c Effects ofthe detection of blank (bufferonly), 10 nmol·L−1 of other metalions, 0.1 nmol·L−1 Hg2+ ions, anda mixture of 0.1 nmol·L−1 Hg2+

ions and 1 nmol·L−1 of each of theother eleven metal ions, in a0.1 mol∙L−1 phosphate buffer(pH 7.4) at the bias voltage of0.1 V following light irradiation(λ = 430 nm); d Time-basedphotocurrent response of theassay under several on/offirradiation cycles for 1000 s

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optimized conditions, the extent of final photocurrent enlarge-ment (ΔI) is proportional to the logarithm of Hg2+ concentra-tion, and the equation of the calibration curve is Y = 12.05 +5.88X (R = 0.996). A linear dynamic range is observed from10 pmol·L−1 to 100 nmol·L−1 and a low detection limit (LOD)of 5 pmol·L−1 is gotten. This result is well below the maxi-mum allowable levels of Hg2+ in drinking water regulated byWHO (i.e. 30 nmol·L−1) and EPA (i.e. 10 nmol·L−1).

Furthermore, to evaluate the advancement of this work, anoverview on recently reported nanomaterial-based methodsfor determination of Hg2+ using T-Hg-T interaction existingmethod is summarized in Table 1. As shown in Table 1, elec-trochemical detection of Hg is more mature and get the bestresults. Although the PEC assay for detecting Hg has gotten arelatively good result in this work, there is still deep researchshould be done in future.

The selectivity, reproducibility and stability of the assay

Selectivity is an important criterion for analysis and detec-tion. To demonstrate the photocurrent response originatedfrom specific binding, the photoelectric response to Hg2+ ata concentration of 0.1 nmol·L−1 was evaluated by comparingthe responses to blank (buffer only) and various metal ions,including Ca2+, K+, Ag+, Zn2+, Cu2+, Fe2+, Na, Fe3+, Al3+,Mg2+ and Mn2+, each at a concentration of 10 nmol·L−1.Furthermore, the mixture of all these interfering ions (eachat a concentration of 10 nmol·L−1) and Hg2+ (at a concentra-tion of 0.1 nmol·L−1) was also evaluated. As shown inFig. 4c, only samples containing Hg2+ has clearly and re-markably photocurrent response. Therefore, the PEC assayexhibits excellent selectivity for Hg2+, which is mainly at-tributed to the highly specific interaction of the T-Hg2+-Tcoordination chemistry.

To examine the reproducibility of the PEC assay, the pho-tocurrent responses of the same samples with five modifiedelectrodes prepared independently were investigated. Afteranalyzing from the experimental results, relative standard de-viation (RSD) of 1.87% and less than 5% was obtained. Thisindicates the preferable precision and acceptable reproducibil-ity of the PEC assay.

The stability was also evaluated through observing the pho-tocurrent responses recorded under several on/off irradiationcycles for 1000 s. As shown in Fig. 4d, the PEC assay displaysreproducible photocurrent responses without any noticeablevariation, which indicates the structural stability of the PECassay can be accepted.

Detection of Hg2+ in water samples

To demonstrate the practical application of the PEC assay andevaluate the applicability of this assay to detect Hg2+, samplesof tap water with different concentrations of Hg2+ were ana-lyzed using standard addition methods. Tap water filtered by0.22 mm membrane was spiked with series concentrations ofHg2+ (2.5, 5.0 and 7.5 nmol·L−1) prior to photocurrent intensitydetermination. The recovered Hg2+ concentrations are calculat-ed according to the determined photocurrent intensities and thelinear equation shown in Fig. 4b. As shown in Table S1, theaverage recoveries of the PEC assay are in the range of 98.4–102.4% and RSDwas 3.5–4.1%. The analytical results indicatethe PEC assay to detect Hg2+ in tap water is satisfactory.

Conclusions

In this work, we describe a highly sensitive and selective PECassay for Hg(II) ions detection based on a dual signal

Table 1 An overview on recently reported nanomaterial-based methods for determination of Hg(II) using T-Hg-T interaction

Method Materials Linear range(nmol∙L−1)

LODs(nmol∙L−1)

Reference

Electrochemiluminescent assay Nanoparticles doped with Ru(bpy)32+ 0.001 to 0.009 & 0.01 to 0.09 0.0003 [11]

Electrochemical assay Gold amalgam and silver nanoparticles 0.005 to 80 0.002 [12]

Electrochemical assay Nanoporous gold 0.01 to 5000 0.0036 [34]

Electrochemical assay Gold nanoparticles/reduced graphene oxidenanocomposites

0.05 to 5 0.0075 [35]

Glucometer-based assay Gold clusters 0.05 to 80 0.01 [36]

Fluorometric assay Reduced graphene oxide 100 to 700 5 [37]

Fluorometric assay Ethynyl and 6-carboxyl-fluorescein 5 to 150 0.42 [38]

Fluorometric assay Nano-sized graphene oxide 25 to 200 8.5 [39]

Bare-eye detection Functionalized PDA vesicles 1000 to 100000 100 [40]

Photoelectrochemical assay Au@Ag-MoS2 nanohybrid and CdSequantum dots

0.010 to 0.10 0.005 This work

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amplification strategy. Firstly, the SPR enhanced Au@Ag-MoS2 nanohybrid got a higher photocurrent compared withpure MoS2 NSs, which reduced the background interference.Secondly, the ct-DNA aptamer attached on the film specifical-ly bound to the Hg2+ ion through the formation of T-Hg2+-Tcomplexes, leading to the formation of a hairpin-shaped dou-ble stranded DNA structure. Lastly, after the Hg2+ ions inter-calated in the double-stranded DNA, CdSe QDs were near thesurface of Au@Ag-MoS2, leading to the enhanced photocur-rent response of the assay and realize dual signal amplificationto get high sensitivity. Additionally, the fabricated PEC assaycan be satisfactorily applied to the practical determination ofHg2+. This PEC sensing strategy can be extended for probingother metal ions and biomolecules with similar target-basepairs, and might open a perspective for PEC conversion re-search as well as artificial smart materials.

Acknowledgments This study was supported by the Natural ScienceFoundation of China (No. 21575050), the Natural Science Foundation ofShandong Province (No.ZR2013BL003) and the Doctoral ScienceFoundation of University of Jinan (No. XBS1658).

Compliance with ethical standards The authors declare that they haveno competing interests.

References

1. Coronado E, Galan-Mascaros JR, Marti-Gastaldo C, Palomares E,Durrant JR, Vilar R, Gratzel M, Nazeeruddin MK (2005)Reversible colorimetric probes for mercury sensing. J Am ChemSoc 127:12351–12356

2. Zhao M, Fan G, Chen J, Shi J, Zhu J (2015) Highly sensitive andselective photoelectrochemical biosensor for Hg2+ detection basedon dual signal amplification by exciton energy transfer coupledwith sensitization effect. Anal Chem 87:12340–12347

3. Harris H, Pickering I, George G (2003) The chemical form of mer-cury in fish. Science 301:1203

4. Liu M, Wang ZY, Zong SF, Chen H, Zhu D, Wu L, Hu GH,Cui YP (2014) SERS detection and removal of mercury(II)/silver(I) using oligonucleotide- functionalized core/shellmagnetic silica sphere@Au nanoparticles. ACS Appl MaterInterfaces 6:7371–7379

5. Huang C, Yang Z, Lee K, Chang H (2007) Synthesis of highlyfluorescent gold nanoparticles for sensing mercury(II). AngewChem Int Ed 46:6948–6952

6. Li T, Dong SJ, Wang EK (2009) Label-free colorimetric detectionof aqueous mercury ion (Hg2+) using Hg2+-modulated G-quadruplex-based DNAzymes. Anal Chem 81:2144–2149

7. Zhu ZQ, Su YY, Li J, Li D, Zhang J, Song SP, Zhao Y, Li GX, FanCH (2009) Highly sensitive electrochemical sensor for mercury(II)ions by using a mercury-specific oligonucleotide probe and goldnanoparticle-based amplification. Anal Chem 81:7660–7666

8. Wang LN, Liu FY, Sui N, Liu MH, Yu WW (2016) A colorimetricassay for Hg(II) based on the use of a magnetic aptamer and ahybridization chain reaction. Microchim Acta 183:2855

9. Senthamizhan A, Celebioglu A, Uyar T (2015) Real-timeselective visual monitoring of Hg2+ detection at ppt level:an approach to lighting electrospun nanofibers using goldnanoclusters. Sci Rep 5:10403

10. Zarlaida F, Adlim M (2016) Gold and silver nanoparticles and in-dicator dyes as active agents in colorimetric spot and strip tests formercury(II) ions: a review. Microchim Acta 184:45–58

1 1 . C a i L F , G u o Z H , Z h e n g X W ( 2 0 1 6 )Electrochemiluminescent detection of Hg(II) by exploitingthe differences in the adsorption of free T-rich oligomersand Hg(II) loaded T-rich oligomers on silica nanoparticlesdoped with Ru(bpy)3

2+. Microchim Acta 183:2345–235112. Tang J, Huang YP, Zhang CC, Liu HQ, Tang DP (2016) DNA-

based electrochemical determination of mercury(II) by exploitingthe catalytic formation of gold amalgam and of silver nanoparticles.Microchim Acta 183:1805–1812

13. Zhao W, Xu J, Chen H (2015) Photoelectrochemical bioanalysis:the state of the art. Chem Soc Rev 44:729–741

14. Yang ZQ, Wang FR, Wang M, Yin HS, Ai SY (2015) A novelsignal-on strategy for M.Sss/ methyltransfease activity analysisand inhibitor screening based on photoelectrochemicalimmunosensor. Biosens Bioelectron 66:109–114

15. Fabiana A, Stefano C, Viviana S, Danila M (2016) Nanomaterialsin electrochemical biosensors for pesticide detection: advances andchallenges in food analysis. Microchim Acta 183:2063

16. Ge L, Wang WX, Hou T, Li F (2016) A versatile immobilization-free photoelectrochemical biosensor for ultrasensitive detection ofcancer biomarker based on enzyme-free cascaded quadratic ampli-fication strategy. Biosens Bioelectron 77:220–226

17. Zhang XR, Liu MS, Liu HX, Zhang SS (2014) Low-toxic Ag2Squantum dots for photoelectrochemical detection glucose and can-cer cells. Biosens Bioelectron 56:307–312

18. Chamier J, Leaner J, Crouch A (2010) Photoelectrochemicaldetermination of inorganic mercury in aqueous solutions.Anal Chim Acta 661:91–96

19. Tang Q, Jiang D (2015) Stabilization and band-gap tuning ofthe 1T-MoS2 monolayer by covalent functionalization. ChemMater 27:3743–3748

20. Zang Y, Lei JP, Hao Q, Ju HX (2016) CdS/MoS2 heterojunction-based photoelectrochemical DNA biosensor via enhanced chemilu-minescence excitation. Biosens Bioelectron 77:557–564

21. Scharf T, Goeke R, Kotula P, Prasad S (2013) Synthesis of Au-MoS2 nanocomposites: thermal and friction-induced changes tothe structure. ACS Appl Mater Interfaces 5:11762–11767

22. ZhangY, Shoaiba A, Li JJ, JiMW, Liu JJ, XuM, Tong B, Zhang JT,Wei Q (2016) Plasmon enhanced photoelectrochemical sensing ofmercury (II) ions in human serum based on Au@Ag nanorodsmodified TiO2 nanosheets film. Biosens Bioelectron 79:866–873

23. Shuang S, Lv RT, Xie Z, Zhang ZJ (2016) Surface plasmon en-hanced photocatalysis of Au/Pt-decorated TiO2 nanopillar arrays.Sci Rep 6:26670

24. Shi Y, Wang J, Wang C, Zhai TT, Bao WJ, Xu JJ, Xia XH, ChenHY (2015) Hot electron of Au nanorods activates theelectrocatalysis of hydrogen evolution on MoS2 nanosheets. J AmChem Soc 137:7365–7370

25. Yan K, Wang R, Zhang JD (2014) A photoelectrochemical biosen-sor for o-aminophenol based on assembling of CdSe and DNA onTiO2 film electrode. Biosens Bioelectron 53:301–304

26. Shen QM, Han L, Fan GC, Abdel-Halim ES, Jiang LP, Zhu JJ(2015) Highly sensitive photoelectrochemical assay for DNAmeth-yltransferase activity and inhibitor screening by exciton energytransfer coupled with enzyme cleavage biosensing strategy.Biosens Bioelectron 64:449–455

27. Qiu ZL, Shu J, Jin GX, Xu MD, Wei QH, Chen GN, Tang DP(2016) Invertase-labeling gold-dendrimer for in situ amplified de-tection mercury (II) with glucometer readout and thymine–Hg2+–thymine coordination chemistry. Biosens Bioelectron 77:681–686

28. Zhang Y, Ma HM, Wu D, Li Y, Du B, Wei Q (2015) Label-freeimmunosensor based on Au@Ag2S nanoparticles/magnetic

1386 Microchim Acta (2017) 184:1379–1387

chitosan matrix for sensitive determination of ractopamine. JElectroanal Chem 741:14–19

29. Chou SS, DeM,Kim J, Byun S, Dykstra C, Yu J, Huang JX, DravidVP (2013) Ligand conjugation of chemically exfoliated MoS2. JAm Chem Soc 135:4584–4587

30. Zheng YN, Liang WB, Yuan YL, Xiong CY, Xie SB, Wang HJ,Chai YQ, Yuan R (2016) Wavelength-resolved simultaneousphotoelectrochemical bifunctional sensor on single interface: anewly in vitro approach for multiplexed DNAmonitoring in cancercells. Biosens Bioelectron 81:423–430

31. Wang XX, Nan FX, Zhao JL, Yang T, Ge T, Jiao K (2015) A label-free ultrasensitive electrochemical DNA sensor based on thin-layerMoS2 nanosheets with high electrochemical activity. BiosensBioelectron 64:386–391

32. Qiu ZL, Tang DY, Shu J, Chen GN, Tang DP (2016)Enzyme- t r i gge red fo rma t i on o f enzyme- ty r amineconcatamers on nanogold-functionalized dendrimer forimpedimetric detection of Hg(II) with sensitivity enhance-ment. Biosens Bioelectron 75:108–115

33. Liu L, Xia N, Liu HP, Kang XJ, Liu XS, Xue C, He XL (2014)Highly sensitive and label-free electrochemical detection ofmicroRNAs based on triple signal amplification of multifunctionalgold nanoparticles, enzymes and redox-cycling reaction. BiosensBioelectron 53:399–405

34. Zeng GM, Zhang C, Huang DL, Lai C, Tang L, Zhou YY, Xu P,Wang H, Qin L, Cheng M (2016) Practical and regenerable

electrochemical aptasensor based on nanoporous gold and thy-mine-Hg2+-thymine base pairs for Hg2+ detection. BiosensBioelectron. doi:10.1016/j.bios.2016.10.018

35. Wang N, Lin M, Dai HX, Ma HY (2016) Functionalized goldnanoparticles/reduced graphene oxide nanocomposites for ultrasen-sitive electrochemical sensing of mercury ions based on thymine-mercury-thymine structure. Biosens Bioelectron 79:320–326

36. Zhang J, Tang Y, Lv J, Fang SQ, Tang DP (2015) Glucometer-based quantitative determination of Hg(II) using gold particle en-capsulated invertase and strong thymine-Hg(II)-thymine interactionfor signal amplification. Microchim Acta 182:1153–1159

37. Abdelhamid HN, Wu HF (2015) Reduced graphene oxideconjugate thymine as a new probe for ultrasensitive andselective fluorometric determination of mercury(II) ions.Microchim Acta 182:1609–1617

38. Zhang JR, Huang WT, Zeng AL, Luo HQ, Li NB (2015) Ethynyland pi-stacked thymine-Hg2+-thymine base pairs enhanced fluores-cence quenching via photoinduced electron transfer and simple andsensitive mercury ion sensing. Biosens Bioelectron 64:597–604

39. Huang PJ, van Ballegooie C, Liu J (2016) Hg2+ detection using aphosphorothioate RNA probe adsorbed on graphene oxide and acomparison with thymine-rich DNA. Analyst 141:3788–3793

40. Ma X, Sheng ZH, Jiang L (2014) Sensitive naked-eye detection ofHg2+ based on the aggregation and filtration of thymine function-alized vesicles caused by selective interaction between thymine andHg2+. Analyst 139:3365–3368

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