Research Article Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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Enhancing the Photoelectrochemical Response of DNA Biosensors Using Wrinkled Interfaces Sudip Saha,† Yuting Chan,‡ and Leyla Soleymani*,†,‡ †
School of Biomedical Engineering and ‡Department of Engineering Physics, McMaster University, 1280 Main Street West, Hamilton, Ontario L8S 4L7, Canada
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ABSTRACT: Photoelectrochemical (PEC) biosensors, with optical biasing and electrochemical readout, are expected to enhance the limit-of-detection of electrochemical biosensors by lowering their background signals. However, when PEC transducers are functionalized with biorecognition layers, their current significantly decreases, which reduces their signal-to-noise ratio and dynamic range. Here, we develop and investigate a wrinkled conductive scaffold for loading photoactive quantum dots into an electrode. The wrinkled photoelectrodes demonstrate an order of magnitude enhancement in the magnitude of the transduced PEC current compared to their planar counterparts. We engineer PEC biosensors by functionalizing the wrinkled photoelectrodes with nucleic acid capture probes. We challenge the sensitivity of the wrinkled and planar biosensors with various concentrations of DNA target and observe a 200 times enhancement in the limit-of-detection for wrinkled versus planar electrodes. In addition to enhanced sensitivity, the wrinkled PEC biosensors are capable of distinguishing between fully complementary and targets with a single base-pair mismatch, demonstrating the suitability of these biosensors for use in clinical diagnostics. KEYWORDS: photoelectrochemistry, biosensing, DNA sensing, wrinkling, quantum dots, PEC sensing, shrink film, nanoparticles
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INTRODUCTION In photoelectrochemistry, light is used to generate electron/ hole pairs in a photoactive material, and these electron/hole pairs, when separated, are used to drive redox reactions.1 Depending on the reactions occurring in the photoelectrochemical (PEC) cell, light is then converted to electrical or chemical energy. Regardless of the type of the PEC cell photovoltaic, photosynthetic, or photocatalytic2increasing the light-harvesting efficiency is crucial for enhancing the overall energy conversion efficiency. Recently, PEC signal transduction has been demonstrated for biological sensing.3 In PEC biosensors, light is used to generate charge carriers in photoactive materials, and the transduced electrochemical current is measured for analyzing biologically relevant targets.4 Because signal readout is electrochemical, this method inherits the benefits of electrochemical biosensing: the signal is read using inexpensive5 and easy-to-use instrumentation, and multiplexed detection is achieved using multielectrode microchips.6 Due to optical excitation, PEC measurements are performed at lower bias potentials compared to their electrochemical counterparts.7 This lowers the measured electrochemical background currents and increases the signal-to-background ratio.8 PEC readout has been used to detect biomolecules such as DNA,3 RNA,9 and proteins.10−13 In biomolecular sensors with PEC readout, the surface of the photoactive material is functionalized with biorecognition layers, such as nucleic acids,14 antibodies, or aptamers.3,15 This biofunctionalization significantly reduces the PEC signal magnitude by limiting the © XXXX American Chemical Society
access of redox or electron donor/acceptor species to the photoactive material.16 As a result, similar to the conventional PEC cells, enhanced light absorption and extraction efficiency are of paramount importance in sensing cells. Implementing micro/nanostructures on the surface of photoactive electrodes significantly improves the light absorption efficiency17,18 due to increased particle density,19 internal scattering,20 and increased optical path length of the incident light.21 Lithography,22 electrochemical processing,23,24 chemical etching, and energetic beam-based treatment25−27 are widely used in implementing surface structuring. However, when using these fabrication methods, a trade-off must be made between the degree of structural tunability, throughput, and cost. Surface wrinkling is a facile and inexpensive method28 for introducing tunable micro- and nanostructuring23 into thin films,29 porous networks,30 and assembly of nanoparticles.28,31,32 Wrinkling occurs when a compliant substrate modified with a stiff skin is exposed to compressive in-plane strain31 or when the substrate is subjected to the removal of tensile strain.33 The mismatch in the elastic moduli of the stiff layer and the compliant substrate results in the formation of wrinkles. The amplitude and wavelength of wrinkles are tuned by varying the layer thickness, the layer and substrate mechanical properties, and the amount of induced strain.31 Recently, surface wrinkling has been explored for enhancing Received: July 20, 2018 Accepted: August 27, 2018
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DOI: 10.1021/acsami.8b12286 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
Figure 1. Fabrication of QD-based photoelectrodes. (a) Planar photoelectrodes were fabricated by sputtering ITO on polystyrene in the figure and layer-by-layer assembly of CdTe QDs. (b) Wrinkled photoelectrodes were fabricated by sputtering ITO, heat shrinking polystyrene at 140 °C, and layer-by-layer assembly of CdTe QDs. (c) Photoelectrodes with a wrinkled scaffold were fabricated by oxidizing the surface of polystyrene using UV/ozone (UVO) treatment, heat shrinking polystyrene at 140 °C, sputtering ITO, and layer-by-layer assembly of CdTe QDs.
Figure 2. Comparison of photoelectrodes with different structures. From left to right: the schematic illustration, low-magnification scanning electron micrograph, high-magnification scanning electron micrograph, and cross-sectional scanning transmission electron micrograph of a (a) planar electrode created by depositing CdTe on a planar ITO electrode, (b) wrinkled electrode created by depositing CdTe on a wrinkled ITO electrode, and (c) scaffolded-wrinkled photoelectrodes created by depositing ITO and CdTe on a wrinkled oxidized polystyrene scaffold.
the light extraction efficiency of photovoltaic devices.34,35 In a photovoltaic device with an organic semiconductor used as the active layer, the amount of light absorption with infrared illumination increases from
