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Enhanced Photocatalysis and Biomolecular Sensing with Field-Activated Nanotube-Nanoparticle Templates

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Enhanced Photocatalysis and Biomolecular Sensing with Field-Activated Nanotube-Nanoparticle Templates ARTICLE https://doi.org/10.1038/s41467-019-10393-9 OPEN Enhanced photocatalysis and biomolecular sensing with field-activated nanotube-nanoparticle templates Sawsan Almohammed 1,2, Sebastian Tade Barwich3, Andrew K. Mitchell 1, Brian J. Rodriguez 1,2 & James H. Rice1 1234567890():,; The development of new catalysts for oxidation reactions is of central importance for many industrial processes. Plasmonic catalysis involves photoexcitation of templates/chips to drive and enhance oxidation of target molecules. Raman-based sensing of target molecules can also be enhanced by these templates. This provides motivation for the rational design, characterization, and experimental demonstration of effective template nanostructures. In this paper, we report on a template comprising silver nanoparticles on aligned peptide nanotubes, contacted with a microfabricated chip in a dry environment. Efficient plasmonic catalysis for oxidation of molecules such as p-aminothiophenol results from facile trans- template charge transfer, activated and controlled by application of an electric field. Raman detection of biomolecules such as glucose and nucleobases are also dramatically enhanced by the template. A reduced quantum mechanical model is formulated, comprising a minimum description of key components. Calculated nanotube-metal-molecule charge transfer is used to understand the catalytic mechanism and shows this system is well-optimized. 1 School of Physics, University College Dublin, Belfield, Dublin 4 D04 V1W8, Ireland. 2 Conway Institute of Biomolecular and Biomedical Research, University College Dublin, Belfield, Dublin 4 D04 V1W8, Ireland. 3 School of Physics, Trinity College Dublin, Dublin 2 D02 PN40, Ireland. Correspondence and requests for materials should be addressed to A.K.M. (email: [email protected]) or to B.J.R. (email: [email protected]) or to J.H.R. (email: [email protected]) NATURE COMMUNICATIONS | (2019) 10:2496 | https://doi.org/10.1038/s41467-019-10393-9 | www.nature.com/naturecommunications 1 ARTICLE NATURE COMMUNICATIONS | https://doi.org/10.1038/s41467-019-10393-9 any industrial processes require efficient catalysis of (exemplified here by p-aminothiophenol (PATP) oxidized to p- redox reactions1–4. A seemingly different problem is the nitrothiophenol (PNTP), and 2-aminophenol (2-AMP) oxidized M fi sensing of biomolecules, which can pose its own chal- to 2-nitrophenol (2-NIP)), exploiting the facile eld-activated lenges4–6. In fact, these scientific problems are related—both trans-template charge transfer. We also demonstrate that this catalytic activity and sensing can be dramatically enhanced by the same approach can be used to enhance the strength of Raman use of nanostructured templates/chips. In the field of plasmon scattering from molecules with small Raman cross-sections for catalysis, a major goal of current research is to design such example glucose and DNA-based molecules, establishing the templates to facilitate charge transfer to target molecules, thereby potential of our template design for sensitive detection and ana- enhancing plasmonic photocatalytic activity1–4 and enhancing lytics. This approach is versatile and can be applied to a range of the Raman signal intensity of otherwise spectroscopically dark plasmonic metal nanoparticle and semiconductor combinations. molecules4–6. Nanomaterials that exhibit elevated catalytic performance play a Results fi signi cant role in high value industrial processes such as the Photocatalysis studies. The template comprises microfabricated 1–4 reduction of carbon dioxide or nitroaromatics . Catalytic pro- gold electrodes on an Si substrate, with a 0.1 mm opening size cesses augmented by plasmon active nanomaterials for example between electrodes. FF-PNTs are aligned during self-assembly on an gold (Au) or silver (Ag) offer significant potential to enhance – optically-thick insulating layer of SiO2 1 mm wide, exploiting heterogeneous catalytic processes4 6. The catalytic enhancement 21,22 Si/SiO2 wettability differences , and Ag NPs deposited (Fig. 1a, associated with metal nanoparticles (NPs) results from optical Supplementary Fig. 1). Experiments are undertaken in a dry excitation of localized surface plasmon resonances (LSPRs) that environment (not electrochemical). Scanning electron microscopy act as the energy input to drive chemical transformations in redox (SEM) images (Fig. 1b–dandSupplementaryFig.2)showthe reactions, and may also be facilitated by the intense local electric alignment of the FF-PNT and the topology/morphology of depos- fi 1–4 fi eld and local heat generated . The speci c mechanisms of ited Ag NPs, which aggregate around the FF-PNTs due to LSPR-mediated photocatalytic processes are not fully understood, functional-group interactions21,22. An external electric field is pro- but are investigated in detail in this paper. It is widely believed that duced by applying a voltage across the electrodes. The current fi hot electron processes play a signi cant role in the transferal of measured through the FF-PNT/Ag NPs template due to the applied chemical, photon, and electrical energies during plasmon- voltage reveals Ohmic behavior (Supplementary Fig. 3), with a 2–4 fl mediated catalysis . The correlation between hot electron ow resistance (658 ± 0.77 Ω) lower than for Ag NPs (1583 ± 0.92 Ω)or and the rates of photocatalytic reactions has been reported in for FF-PNTs (930 ± 1.03 Ω) alone, consistent with previous fl several studies revealing that hot electrons strongly in uence studies23. These results further demonstrate the feasibility of using 2–6 heterogeneous photocatalysis . Through setting the wavelength FF-PNTs in electronic devices, and provide a platform to explore – of incident radiation to match the HOMO LUMO gap of attached the influenceonSERSofanappliedelectricfield. molecules, charge transfer to the molecule can occur, giving rise to We first investigate the impact of applying an electric field on oxidation reactions in certain molecules or enhanced Raman the oxidation reaction PATP → PNTP using the FF-PNT/Ag NP scattering cross section for sensing in other molecules. template, as monitored by SERS using an excitation wavelength Nanocomposites combining semiconductor materials with metal fixed at 532 nm. The Raman and SERS spectra for PATP fi 6–9 NPs bring additional bene ts to plasmonic photocatalysis . (Supplementary Fig. 4) is well characterized and is known to The contact potential difference between metal NPs and a semi- undergo photochemical changes in the presence of plasmonic 6–9 conductor can separate photogenerated electrons and holes , metals24–27. The electric field is generated by applying a voltage thereby reducing electron-hole pair diffusion lengths and leading across the gold nanoelectrodes (see Methods). For an offset bias fi to more ef cient photogenerated charge separation and transfer, of <~100 mV, a seven-fold increase in SERS signal intensity was 2–5 which in turn enhances photocatalytic activity . observed, but with no change in the relative positions of the SERS fi Here, we show an increase in probability and ef ciency of spectral peaks (Supplementary Fig. 5). However, with an offset both chemical reactions and SERS detection through electro- bias >~100 mV, the eight-fold increase in SERS signal strength optical synergy, using a microfabricated chip design (in air was accompanied with differences in the vibrational mode rather than electrochemical)2–4. This is achieved through the use of a plasmonic-semiconductor system based on aligned diphe- ab nylalanine peptide nanotube (FF-PNTs) wide band gap semi- conductors10–15. Diphenylalanine (FF), a peptide consisting of a naturally occurring amino acid phenylalanine, can self-assemble into micro and nanosized tubular structures10–17. The resulting organic self-assembled nanotubes are the FF-PNTs, which are FF-PNT/Ag NP c stable/robust, rigid, and also biocompatible. They can be used SiO2 in applications requiring the use of a wide bandgap semi- Raman Gold 10–17 conductor . FF-PNTs have been reported to have high ther- spectroscopy Si substrate mal and chemical stability10–15 in addition to piezoelectric16,17 and pyroelectric15 properties. We show experimentally and the- oretically that applying a longitudinal electric field allows the FF- d PNT density of states to be tuned from a semiconductor to a metal, enabling effective charge transfer from the nanotube to the metal nanoparticles. This results in an enhancement in the state density of hot electrons18–20. The effect is optimized through the physical alignment of the FF-PNTs, since the inherent electric dipoles of the FF-PNTs are then aligned and maximally Fig. 1 Substrate fabrication and characterization. a Schematic sketch of the responsive to the applied longitudinal electric field (a cooperative template design that is used in the study. b-d Scanning electron microscopy effect). We demonstrate that this optoelectrical device enhances images of the aligned FF-PNT/Ag NP structures located between the gold photocatalytic conversion for model oxidation reactions electrodes 2 NATURE COMMUNICATIONS | (2019) 10:2496 | https://doi.org/10.1038/s41467-019-10393-9 | www.nature.com/naturecommunications NATURE COMMUNICATIONS | https://doi.org/10.1038/s41467-019-10393-9 ARTICLE ab10 140 AC @ 40 kHz; PATP 140 + 6 V @ 40 kHz PATP PATP 9 160 200 mV offset PNTP DMAB 120 + + 120 + 8 120 DMAB DMAB + PNTP 7 100 80 100 PNTP 6 40 80 0 80 5 0200400 60 60 4 Offset (mV) 3 (V) Voltage 40 Intensity (a.u.) Intensity (a.u.) 40 2 20 AC @ 40 kHz; 20 1 200 mV offset 0 0 600 800 1000 1200 1400 1600 0246810 Raman shift (cm–1) Voltage (V) c 140 d 1.0 AC @ 40 kHz; 15 V, 350 mV offset 120 Control (zero field) 100 0.8 80 0.6 60 0.4 40 Intensity (a.u.) 0.2 20 Normalized-intensity 0.0 0 0 10 20 30 40 50 60 600 800 1000 1200 1400 1600 Voltage (V) Raman shift (cm–1) e f 140 SH SH AC @ 10 V; 120 300 mV offset 100 80 60 AC @ 4 V; Intensity (a.u.) 100 mV offset 40 20 NH2 NO2 0123456 PATP PNTP Cycle Fig. 2 Influence of applied electric field on the SERS spectra of PATP on the template.
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