Photoelectric conversion switch based on quantum

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Photoelectric conversion switch based on quantum dots with i-motif DNA .... the Au Fermi level is below the electron level of CdSe/ZnS. QDs, tunneling of the ...
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Photoelectric conversion switch based on quantum dots with i-motif DNA scaffoldsw Haifeng Meng,a Yang Yang,a Yingjuan Chen,a Yunlong Zhou,a Yaling Liu,a Xin’an Chen,c Hongwei Ma,c Zhiyong Tang,*a Dongsheng Liu*a and Lei Jiang*b Received (in Cambridge, UK) 17th February 2009, Accepted 20th March 2009 First published as an Advance Article on the web 27th March 2009 DOI: 10.1039/b903325d Combining the best features of both inorganic quantum dots and i-motif DNA, a dynamic pH-driven modulation system of photoelectric conversion was realized by making use of their conjugates immobilized on a Au electrode. Photosynthesis in plants and some types of bacteria, a process of converting solar energy to chemical energy, is responsible for feeding almost all living things on Earth. Enlightened by this bioprocess, the key to realizing controllable photoelectric conversion is recognized as rational design of the light absorption center and the electron transport route. Recently, inorganic quantum dots (QDs) have been selected as the light absorption center due to their advantages over conventional organic pigments, e.g., broad and large absorption profiles, tunable emissions, and high photostability.1 QDs have been extensively used in research on photo-induced electron transfer2 and Fo¨rster resonance energy transfer (FRET)3 which is expected to develop the next generation of solar cells4 or biosensors.5 Moreover, DNA-cross-linked CdS nanoparticles (NPs) have also been constructed to generate photocurrent under illumination.6 However, dynamic modulation of the photoelectric conversion in devices is still a big challenge to be conquered. Herein, we design a biomimetic photoelectric conversion system by immobilizing CdSe/ZnS core–shell QDs and functionalized i-motif motor DNA on the surface of a gold electrode (Scheme 1). The sequence of motor DNA used here is 5 0 -SH-TGTTTTCTTCCCTAACCCTAACCCTAACCC-NH2-3 0 , which can fold into a quadruplex i-motif structure in a weak acidic environment, as a consequence of the formation of intramolecular noncanonical base pairs between cytosine (C) and protonated cytosine (CH1).7 While in a weak basic environment, the DNA motor turns back to a single strand, and can hybridize with its complementary strand (CS) to form an extended and rigid double-helix structure. Hence, introduction of motor DNA can reversibly control the

electron transport distance between QDs and the Au surface, and consequently a reversibly dynamic photoelectric conversion switch driven by pH value is achieved. Through combining the best features of both QDs and an i-motif DNA motor, this work provides a new idea in interdisciplinary science and indicates its potential applications in photovoltaic devices and functional smart materials. Water-soluble, highly luminescent, and mercaptopropionic acid (MPA) stabilized CdSe/ZnS core–shell QDs with diameters of around 3 nm were first synthesized following the procedure described in the literature (Fig. S2w).8 Subsequently, by making use of strong thiol–gold interaction followed by classical EDC/NHSS cross-linking reaction, the Au electrode surface and CdSe/ZnS QDs were bridged through mercapto (–SH) and amino (–NH2) ends of DNA molecules, respectively (details in ESIw). X-ray photoelectron spectra (XPS) (Fig. S5 and Table S2w) confirmed the successful immobilization of CdSe/ZnS QDs onto the Au electrode surface through DNA molecules. Further studies using an on-line quartz crystal microbalance (QCM) revealed that the surface coverage of immobilized i-motif DNA molecules and CdSe/ZnS QDs on Au surfaces was about 1.8  1013 strands cm2 and 2.3  1013 particles cm2, respectively (Scheme S1 and Tables S3 and S4w), which was consistent with the theoretical calculated results. As the diameter of the i-motif structure is 1.9 nm,7b theoretical calculation demonstrates that densely packed monolayer coverage of the folded motor DNA is about 2.8  1013 strands cm2. Hence, motor DNA was ascertained to be adsorbed on the Au surface in a submonolayer, and almost every motor DNA molecule was successfully conjugated with one QD in our experiment. The photoelectric measurements were conducted using the prepared QDs–motor DNA–Au conjugates as working electrodes (Fig. S3w). As the current–time (i–t) curves in Fig. 1 show, anodic photocurrents were observed while the

a

National Center for Nanoscience and Technology, Beijing 100190, P.R. China. E-mail: [email protected]; [email protected]; Fax: þ86 10-62656765; Tel: þ86 10-82545580 b Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, P.R. China. E-mail: [email protected]; Fax: þ86 10-82627566; Tel: þ86 10-82621396 c Department of Biomedical Engineering, College of Engineering, Peking University, Beijing 100871, P.R. China w Electronic supplementary information (ESI) available: Details of the DNA sequence, characteristics of CdSe/ZnS QDs, QCM measurements and some spectral data. See DOI: 10.1039/b903325d

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Scheme 1 Scheme of the QDs–motor DNA–Au conjugating electrode (not to scale). The electron transfer process from photoexcited QDs to the Au electrode is modulated by the motor DNA’s conformation change which is driven by changing the pH value of the electrolyte.

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Fig. 1 Photoelectric measurements of motor DNA based devices in PBS (different pH). The black line exhibits the i–t curve of the situation when pH ¼ 5.0, while red and blue ones correspond to pH 8.0 without CS and with CS, respectively. The green line stands for the i–t curve after the pH value is changed from 8.0 back to 5.0.

light was switched on, indicating that the photoexcited electrons transferred from QDs to the Au surface.9a In this system, H2O acted as hole-trapper and gave rise to the O2 production9b,c (Scheme 2). Furthermore, it was evident that in phosphate buffer solution (PBS) of pH 5.0, a large switching photocurrent (the difference in photocurrent between light-on and light-off state) of about 10 nA was observed on the electrode, whereas in PBS solution of pH 8.0, the switching photocurrent was dramatically decreased to less than 2 nA, regardless of with (forming duplex structure) or without CS (Fig. 1). That is, a weak acidic environment benefits energy conversion of the device from light energy to electric energy. The photoelectric characteristics of the parallel prepared electrode with parallel control DNA linkers (sequence as listed

Scheme 2 Scheme of the tunneling process between QDs and Au electrode in different states (a and b). (a) Due to the short distance ds–s, the tunneling speed of photoexcited electrons from QDs to Au electrode, r1, is larger than the intraparticle recombination speed of photo-excited electrons and holes, rdec. (b) The tunneling speed, r1, is exponentially decreased with the increase of ds–s, leading to the dominance of intraparticle recombination speed, rdec. (c) Potential energy diagram of this photoelectric system. Electrons transfer from photo-excited CdSe/ZnS QDs to Au electrode, while H2O is acting as hole-trapper and giving rise to oxygen production.

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Fig. 2 Comparison of the switching photocurrent of motor DNA (black) and control DNA (blue) based devices during five cycles of pH oscillation between 5.0 and 8.0. Evidently, the switching photocurrent of the motor DNA device is pH-dependent, but that of the control device is not.

in Table S1w) were also investigated, and the switching photocurrent remained almost unchanged on changing the pH value of the PBS buffer solution (Fig. S6w). Fig. 2 displays a comparison study of switching photocurrent against pH values between the two types of electrode devices containing motor DNA linkers (black curve) and control DNA linkers (blue curve) in five cycles. Compared with the electrode containing control DNA linkers, the pH-dependence of photocurrent switching on the electrodes containing motor DNA linkers was much more evident and such switching behavior could be observed for at least five cycles of pH changes. The dynamic response of the switching photocurrent of the motor device can be explained with Marcus theory.10 Because the Au Fermi level is below the electron level of CdSe/ZnS QDs, tunneling of the photo-excited electrons from QDs to Au can occur if the distance permits. Previous studies confirmed that due to the short distance ds–s, the tunneling speed of photo-excited electrons from QDs to the Au electrode was larger than the intraparticle recombination speed of photo-excited electrons and holes, so that photocurrent was generated (Scheme 2).9a Moreover, according to Marcus theory (simplified as equation k ¼ k0ebr), the rate of electron transfer is found to be inversely-exponentially proportional to the distance between the donor and acceptor.9,10 That is, the electron transport can be controlled by modulating the distance between CdSe/ZnS QDs and the Au electrode surface. As Scheme 1 shows, when the pH value of the solutions is changed from 8.0 to 5.0, motor DNA molecules fold from the stretched state to the quadruplex i-motif structure, which shortens the distance between QDs and the Au electrode surface from 10 nm to 3 nm if DNA molecules are fully extended and vertically aligned. However, it should be pointed out that the real distance between QDs and the Au surface is shorter than the above estimation, because DNA strands always align on the surface at a certain tilt angle.7a As a result, the electron transfer rate from photo-excited QDs to the Au electrode becomes faster, leading to larger photocurrent. The reversible pH-dependent configuration change of the DNA motor and its CS can be confirmed by in situ monitoring of its circular dichroism (CD) spectra (Fig. S1w), whereas the This journal is

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configuration of control DNA remains almost unchanged as the pH value of the solution is changed. The XPS elemental analyses could further confirm the DNA conformation changes on the Au surface of the device containing motor DNA linkers. As Table S2w suggested, the percentage content of Cd element in solution of pH 8.0 was 5.7%, which was evidently larger than that in solution of pH 5.0 (4.3%), implying that CdSe/ZnS QDs were lifted up to be exposed (pH 8.0) or dragged down to be hidden (pH 5.0) with the conformation change of DNA motors. Similarly, the percentage decrease of Au element from 12.8% to 8.3% when the pH value was changed from 5.0 to 8.0 was due to the increased depth for XPS detection. However, the XPS elemental analyses of the control device exhibited almost no changes in PBS with different pH values (data not shown). In conclusion, through combining the broad absorption of CdSe/ZnS core–shell QDs and the pH-sensitive conformation of i-motif motor DNA, which referred to the broadabsorption light center and adjustable distance of the electron transport route, we designed a highly reversible pH-driven photoelectric conversion switch. Through tuning the pH value, the changeable conformation of motor DNA would alter the distance between CdSe/ZnS QDs and the Au electrode, so that a dynamic photoelectric conversion system was achieved. Such switch systems would have potential applications in photovoltaic conversion research as well as artificial smart materials.11 We thank the 100-Talent Program of the Chinese Academy of Sciences (ZYT and DL), the National Science Foundation of China (20571077 for LJ, 20773033 for ZYT, 20725309 for DL) and the National Research Fund for Fundamental Key Projects (2007CB936403 for LJ, 2007CB935902 for DL and 2009CB930401 for ZYT) for financial support. The authors also thank M. Liu, Y. Wu, F. Xia, S. Wang, Y. Gao, T. Hu, and W. Wang for their experimental help.

Notes and references 1 (a) W. C. W. Chan and S. Nie, Science, 1998, 281, 2016; (b) X. Michalet, F. F. Pinaud, L. A. Bentolila, J. M. Tsay,

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