Exciton-Plasmon Interactions between CdS Quantum Dots and Ag

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Exciton-Plasmon Interactions between CdS Quantum Dots and Ag Nanoparticles in Photoelectrochemical System and Its Biosensing Application Wei-Wei Zhao, Pei-Pei Yu, Yun Shan, Jing Wang, Jing-Juan Xu,* and Hong-Yuan Chen State key Laboratory of Analytical Chemistry for Life Science, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, China S Supporting Information *

ABSTRACT: With DNA as a rigid spacer, Ag nanoparticles (NPs) were bridged to CdS quantum dots (QDs) for the stimulation of excitonplasmon interactions (EPI) in a photoelectrochemical (PEC) system. Due to their natural absorption overlap, the exciton of the QDs and the plasmon of Ag NPs could be induced simultaneously. The EPI resonant nature enabled manipulating photoresponse of the QDs via tuning interparticle distances. Specifically, the photocurrent of the QDs could be greatly attenuated and even be completely damped by the generated EPI. The work opens a different horizon for EPI investigation through an engineered PEC nanosystem, and provides a viable mechanism for new DNA sensing protocol.

P

QDs and Au NPs was generated by using the spontaneous emission originated from the radiative decay of the former to activate the latter.32 However, in all these above-mentioned works, Au NPs was used exclusively. Comparing with gold, silver has a dielectric function, εAg(ω), very different from that of gold. Besides, since Ag has a stronger plasmon resonance, Ag-based systems can offer larger influence than those provided by Au assemblies.39 Because the plasmon resonance in silver is so strong, Ag-NPs-based assemblies can demonstrate enhanced properties suitable for optical and sensor applications.39 On the other hand, the strong EPI in general needs a sufficient overlap between the exciton band and the plasmon band.42 Hence another merit of Ag NPs is that its plasmon band fully overlaps with the absorption band of utilized CdS QDs, which, upon optical excitation, will bring about a prompt activation of both the plasmon and exciton and subsequent strong EPI that in turn influence the excitonic responses via straightly pointing into the inside of the QDs. So, of particular interest here is the possibility of integrating ingenious Ag NPs-based EPI into PEC platform to exploit the innovative signaling mechanism for advanced PEC bioassay application. If possible, harnessing this type EPI would in principle allow the operation of PEC devices under new physicochemical principles, the success of which could provide great opportunities for further examination of the EPI from a different technical perspective as well as the implementation of novel PEC devices. With special research attention on the plasmon properties of nanosized noble metals for PEC analysis,32 this letter presents our latest findings from a consecutive exploitation of PEC

hotoelectrochemical (PEC) detection is a newly emerged but dynamically developing analysis technique,1−10 which has attracted substantial research scrutiny for its desirable sensitivity and hence better analytical performances.11−13 Its sensing principle depends on the fact that the electrical signal change could be resulted from various biorecognition or biocatalytic events.14−32 However, as a newly appeared detection technique, the PEC bioassay is still in its infancy. Especially, its signaling mechanism is highly limited to using the altered electron donor concentration or its diffusion efficiency caused by enzyme/affinity reaction to achieve the signal diminution/enhancement.12−31 In these works, the photoactivated excitons were commonly steered to react with solution-solubilized electron donor/acceptor to generate anodic/cathodic photocurrent. Namely, the signaling strategy was established on the direct interfacial electron communication between the photoactive material and ambient environment.1 Exciton−plasmon interactions (EPI) represent a fundamental light-matter interplay that is important for engineering future nanoscale optoelectronic devices. Enormous recent effort has been devoted to its unique creation, clear understanding, and exploitation of possible practical applications.33−39 In some earlier works, EPI utilization has been demonstrated as a feasible analytical basis for biosensing purposes. For instance, modulation of EPI in molecular spring assemblies of CdTe nanowires and Au NPs enables its use for protein detection based on the wavelength shift in photoluminescence (PL).38 Lately, interesting results has also been reported about its serving as new biosensing strategy in electrochemiluminescence (ECL).40,41 Given such interactions associates exquisitely to the exciton states, one can expect its great potential in PEC detection. Very recently, interparticle energy transfer between © 2012 American Chemical Society

Received: January 12, 2012 Accepted: June 22, 2012 Published: June 22, 2012 5892

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technique as an advanced signaling platform for bioassay research.22−32 In this work, Ag NPs was selected to construct the hybrid system with CdS QDs due to their natural overlap in absorption spectra. Experimentally, CdS QDs were immobilized onto the indium tin oxide (ITO) electrode through the layer-by-layer self-assembly of oppositely charged cationic polyelectrolytes poly(diallyldimethylammonium chloride) (PDDA, ζ-potential of +6.27 mV) and thioglycolic acid (TGA) capped CdS QDs (ζ-potential of −16.6 mV). Thereafter, the capture DNA was connected to the electrode via the classic 1-Ethyl-3-(3-dimethylaminopropyl) carbodiimide (EDC)/N-hydroxysuccinimide (NHS) coupling reaction and subsequently blocked by monoethanolamine (MEA), followed by which the final hybridization was performed with the Ag NP-labeled cDNA. Upon light irradiation, both the exciton of the QDs and the surface plasmon resonance (SPR) of Ag NPs could be induced simultaneously, leading to the fast generation of EPI. We hence used the produced EPI to photoelectrochemically adjust the excitonic responses in the QDs. Our preliminary study revealed that, under fixed conditions, the presence of the proximal Ag NPs could significantly impair the original photocurrent intensity of QDs. Further investigation demonstrated that the photocurrent was sensitive to the interparticle distance, which was tuned by the employment of double-stranded (ds) DNA as a rigid spacer. The corresponding mechanism was discussed, and the designed platform was then exploited for the quantitative DNA biosensing. In addition to the PEC investigation of Ag NPs-based EPI for the first time, this work presents the novel signaling mechanism for PEC analytical application and to the best of our knowledge has never been reported. Essentially, the efficient excitation of EPI needs a sufficient spectra overlap between the exciton band and the plasmon band. Figure 1 depicts the typical absorption spectrum of the CdS QDs and Ag NPs. The absorption spectrum implies that the CdS QDs have a broad absorption range that is suitable for employment as photoactive substrate. Meanwhile, the absorption spectrum of the Ag NPs is clearly characterized by the symmetrical plasmon peak of Ag NPs at ∼390 nm. Obviously, the excitonic absorption of the QDs has a large

spectral overlap with the plasmon absorption of the Ag NPs, which would be beneficial to the simultaneous inducement of the exciton and plasmon and thus the subsequent generation of EPI, the explanation for which was proposed schematically as shown in Scheme 1. Scheme 1. Schematic Mechanism of the Operating PEC Systema

a

Major processes involves photon absorption and electron excitation from the valence band (VB) to the conduction band (CB) to generate e−−h+ pair; hole scavenging by electron donor (d); electron transfer (eT) which collected by the electrode for electronic readout; e−−h+ recombination consisted of nonradiative decay (nD) and radiative decay (rD) with spontaneous emission; EPI between Ag NP and CdS QD. The whole event is transduced into a photocurrent signal for analysis.

As illustrated, the interparticle dsDNA bridging brings about the Ag NPs in the immediate vicinity of CdS QDs. Upon light irradiation, on the one hand, the photoexcitation of the QDs leads to the excitonic responses in the QDs, namely, electron− hole (e−−h+) pair generation. Once the process occurred, the e−−h+ pair would be destined for recombination (radiative decay (rD) or nonradiative decay (nD)) or spatial electron transfer (such as eT in the Scheme 1). To suppress the corrosion (lattice dissolution) of CdS QDs under illumination and to facilitate the generation of stable photocurrent signal, electron donor is commonly needed in the PEC platform. On the other hand, SPR of the Ag NPs would be stimulated due to the collective oscillations of conduction electrons driven by the applied electromagnetic field of incident light.43 As the EPI happens in the PEC CdS QDs/Ag NPs system, the excitonic response in QDs could be modulated greatly.38,39 Specifically,

Figure 1. UV−vis absorption spectrum of the prepared CdS QDs (red) and synthesized Ag NPs (blue) in aqueous solution. 5893

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that the local electric field and the radiative rate could be increased by factors of 140 and 1000, respectively, near an Ag particle.47 Actually, the e−h recombination was very fast in original CdS QDs,48 and the effect of EET was very efficient at short interparticle distance.49 After the introduction of EPI into the system, the sharp photocurrent diminution should be attributed to enhanced e−h recombination and hence inefficient photocurrent generation, which caused by the rebalance of PE and EET, as well as other factors involved. Thus, we concluded that, because of the presence of proximal Ag NPs, the enhanced e−h pair recombination should be responsible for the signal reduction primarily. The total separation of these factors seemed also impossible in present PEC platform. However, by varying the experimental parameters, the in-depth understanding of the exact mechanism is currently in progress in our laboratory. Incidentally, the plasmonic photoelectrochemistry of Ag or Au NPs deposited on semiconductor film has also been a very hot topic drawing much research interests recently. The function of such technique relies on the fact that visible light-induced charge separation could occur at the surface of noble metal NPs caused by localized surface plasmon resonance (LSPR) stimulation, leading to charge-transfer photochemistry from noble metal NPs to semiconductors.29,50,51 Thanks to the natural spectra overlap to a large extent, one useful finding is that the present system possesses high flexibility in the selection of irradiation wavelength, indicating its undemanding requirement for future instrumentation. The effect of irradiation wavelength on the electronic readout of photocurrent was then investigated via performing the experiments under the same conditions except the irradiation wavelength variation. Experimental results demonstrated the rather stable strong quenching in photocurrent of the QDs as a function of the excitation wavelength for fixed interparticle spacing. As shown in Figure 3, in the irradiation wavelength ranging from 300 to 500 nm, the system showed a prominent signal reduction with 4 nm interparticle distance (the length of 3 base pairs was considered as 1 nm in this work). Longer and shorter wavelength went against the absorbance of CdS QDs because it could not be activated effectively at longer wavelength and the light intensity of the Xe lamp of the

the proximal Ag NPs influenced the exciton states in CdS QDs via EPI, which involved the contributions from two complicated processes: a plasmon-assisted increase in rD cycling (electromagnetic field or plasmon enhancement, PE) and meanwhile the introduction of nD route through exciton energy transfer (EET). The equilibrium between the PE and EET dictated the final effect of excitonic response, of which the mechanism has not yet fully understood.37,44−46 In the related investigations in PL or ECL, since the optical signal was employed as analytical signal, the process rD and nD were two competitive processes of research focus, and considerable efforts had been dedicated to sorting out their individual contributions on the basis of the enhanced or quenched signals. In fact, the separation of these factors is very difficult for particles in integrated systems or embedded in controlled assemblies.44−46 In contrast, in the present case, because the electronic signal was conducted as readout and both process rD and nD were bound to recombination, the two processes became cooperative and their synergetic effect contended with the process of eT. It means that what needed to be determined here was just their overall effect, the alternation of which could easily be monitored by the photocurrent. Figure 2 depicts the stepwise

Figure 2. Photocurrent intensity in 0.10 M PBS containing 0.10 M ascorbic acid of (a) PDDA/CdS modified ITO electrode, (b) modified with 20 μL, 1 μM capture DNA and blocked by MEA, (c) hybridized with Ag NPs-labeled target DNA. The oligonucleotide sequences of 12 base pairs were used here and its concentration was 1.0 × 10−6 M. The working potential was 0.0 V and the excitation wavelength was 420 nm.

photocurrent responses corresponding to progressive modified electrodes under the irradiation of 420 nm. After anchoring capture DNA and blocking by MEA, the photocurrent decreased ∼8% as compared to the initial intensity, which can be discerned from curves a and b, respectively. This decrement should be attributed to the enhanced steric hindrance of the modifiers, which impeded the diffusion of electron donor (ascorbic acid, AA) to the surface of CdS QDs.4 Noteworthily, after Ag NP-labeled cDNA was hybridized with the capture DNA, an intensive decrease ∼70% of photocurrent intensity was observed as shown by curves b and c in Figure 2. Under present case, the photoconversion efficiency was ∼3.7% and about 3.3 times on the EPI quenched CdS QDs. As compared to Au NPs,32 this stronger quenching effect might relate with the unique property of Ag. It has been estimated

Figure 3. Photocurrent responses before (dark) and after (red) hybridization with the change of irradiation wavelength with 0.10 M AA, 0.0 V working potential and oligonucleotide sequences of 12 base pairs (concentration was 1.0 × 10−6 M). When excitation wavelength was changed, the other conditions were fixed. 5894

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much faster than the PE. At a certain distance, balanced with other factors, the enhanced QD quenching would exhibit a maximum. Further increase in the interparticle distance could result in a decrease of quenching effect.49 In addition, it is worth noting that, as compared to all those pure charge transfer-based PEC systems that difficult to achieve the entire “signal-off”, to the best of our knowledge, the protocol presented here was the only example that resorting to the EPI route which could successfully realize the ∼100% signal damping. This efficient control of exciton states in QDs might be potentially useful for the fabrication of devices for future applications.52 The QDs in this developed system might be exploited as EPI-based PEC transduction elements for various biorecognition events or biocatalytic processes. In such system, QDs are not just passive PEC functional material acting as solid support for the construction of a biosensing interface; they meanwhile play an active role in the transduction of the biorecognition events via interparticle EPI. In the above model work, DNA hybridization has been employed for bridging the Ag NPs to the QDs, switching on the EPI and suppressing the electron transfer process. Since the amount of Ag NPs and the extent of EPI were directly related with the concentration of target DNA, by tracking the transduction signal that monitors the extent of hybridization a new DNA biosensor can be tailored. As shown in Figure 5, with the increase of the concentration of labeled

instrument severely decreased at shorter wavelength. Obviously, as long as in the range of efficient plasmon absorption of Ag NPs, the present system had excellent elasticity in the optical excitation. The high adaptability of the irradiation source, coupled with the electronic readout, would further make its instrumentation simpler, lower cost and easier to miniaturize than that of the optical method, such as PL and ECL. Moreover, because of the total separation and different energy form of excitation source (light) and detection signal (current), this PEC technique could avoid some problems such as the scattered light in the PL technique. Comparing with ECL exploitation, another advantage of the present PEC strategy was that the broad absorption spectra rather than emission bands of QDs was utilized, beneficially exempting the complicated spectra adjustment steps.40 Given the fact that the distance from the fluorophore/ECL emitter to the metal NPs is a critical factor in successful fluorescence/ECL amplification, another question worthy of exploitation here is about the effect of interparticle distance on the PEC signaling. The experiment was designed to control the interparticle distance from 4 to 16 nm by using DNA sequences of 12, 24, 36, and 48 base pairs as rigid spacers. As shown in Figure 4, the extent of final photocurrent reduction increased

Figure 4. Variance ratio of photocurrent caused by distance between CdS QDs and Ag NPs from 4 to 16 nm with 420 nm irradiation light, 0.10 M AA and 0.0 V working potential. When the length of oligonucleotide sequence (concentration was 1.0 × 10−6 M) was changed, the other conditions were fixed. ΔI = I0 − I, I0 and I are the photocurrents of capture DNA/CdS/ITO electrode prior to and after hybridization, respectively.

Figure 5. Effect of different concentrations of target DNA on the differential photocurrent responses (ΔI = I0-I, I0 and I are the photocurrents of capture DNA/CdS/ITO electrode before and after hybridization). Insert: the corresponding calibration curve. The used DNA was 36 bases and the photocurrent measurement was carried out in 0.10 M PBS containing 0.10 M AA. The working potential was 0.0 V and the light wavelength was 420 nm.

gradually with the growth of the DNA length and reached the climax from ∼70% of 12 base pairs to ∼100% of 36 base pairs. When DNA length further increased to 48 base pairs, the quenching effect tended to level off. It is demonstrated for the first time that the PEC signal intensity was sensitive to the interparticle distance between the EPI pairs and hence the PEC signaling could be adjusted accordingly. This phenomena has been commonly reported in the field of PL and it associated intimately with the distance-dependent properties of the EET and PE effects.40,41,44−46,49 Although many factors were involved in such PEC-EPI coupling process, the main quenching effect should be ascribed to the EET, which is a short-range effect and it would be weakened with distance

target DNA, photocurrent decreased correspondingly. The photocurrent decrease was proportional to the concentration of labeled target DNA in logarithmic scale with the linear range from 2.0 × 10−15 to 2.0 × 10−11 M. To confirm the specific interaction between the complement strands, in control experiment the capture DNA modified on the electrode was incubated with the noncomplement DNA and only 2% decrease of the photocurrent intensity was found, indicating the biosensor was highly selective and only responsive to the complement strand. 5895

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In conclusion, Ag NPs was coupled to CdS QDs for the EPI generation in a PEC platform. Due to the natural overlap of their absorption spectra, the exciton and the plasmon could be induced simultaneously, resulting in the photoresponse of the QDs greatly attenuated by the stimulated EPI. We further demonstrated that this system exhibited excellent elasticity in irradiation source and provided convenience for future instrumentation. The EPI resonant nature enabled tailoring of photocurrent manipulation by tuning interparticle distances, and complete PEC signal quenching could be realized for the first time. These results not only opened a different horizon for future EPI investigation through a judiciously engineered PEC nanosystem, but also offered a viable mechanism for new DNA sensing protocol. More generally, integrated PEC biological system with altered EPI could serve as a sensing basis for other biorecognition events or biocatalytic transformations.

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ASSOCIATED CONTENT

S Supporting Information *

The DNA Oligonucleotides Sequences, experimental procedure and supplementary SEM images. These materials are available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel/Fax: +86-25-83597294. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the 973 Program (2012CB932600), the National Natural Science Foundation (No. 21025522 and 21135003) and the National Natural Science Funds for Creative Research Groups (21121091) of China. The authors also thank the referees for their useful comments.

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