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Minor structural changes in transition metal dichalcogenides can have dramatic effects on their electronic properties. This makes the quest for key parameters that enable a selective choice between the competing metallic and semiconducting phases in the 2D MoTe2 system compelling. Herein, we report the optimal conditions at which the choice of the initial seed layer dictates the type of crystal structure of atomically-thin MoTe2 films grown by chemical vapour deposition (CVD). When Mo metal is used as a seed layer, semiconducting 2H-MoTe2 is the only product. Conversely, MoO3 leads to the preferential growth of metallic 1T′-MoTe2. The control over phase growth allows for simultaneous deposition of both 2H-MoTe2 and 1T′-MoTe2 phases on a single substrate during one CVD reaction. Furthermore, Rhodamine 6G dye can be detected using few-layered 1T′-MoTe2 films down to 5 nM concentration, demonstrating surface enhanced Raman spectroscopy (SERS) with sensitivity several orders of magnitude higher than for bulk 1T′-MoTe2. The polymorphism of MoTe2 can be used to realize planar metallic/semiconducting homojunctions in 2D devices, greatly reducing the contact resistance. Here, the simultaneous growth of both phases is achieved on the same substrate by single-step chemical vapor deposition and seeding layer engineering.

Surface enhanced Raman Scattering (SERS) can be used as a novel way of probing local liquid composition and structure at solid-liquid interfaces. This is particularly important for understanding the mechanism of heterogeneous nucleation from solution, where solutes are present at relatively high concentrations. To obtain information about the solution composition and structure near a gold nanoparticle (AuNP) surface, which facilitates SERS, it is thus necessary to understand the role of the analyte in AuNP aggregation, and its effect on the SERS signal and the Raman signal from the bulk. We have used dynamic light scattering and UV-Vis spectroscopy to investigate how glycine influences the aggregation of citrate-functionalised gold nanoparticles (AuNPs), and thus the SERS response, in glycine aqueous solutions. At pH 4 the AuNP suspensions in aqueous solutions (without glycine) did not aggregate due to the electrostatic stabilisation by negatively charged citrate functional groups. However, the addition of glycine promoted aggregation of the AuNPs, concomitantly increasing the strength of the SERS signal. Under these conditions glycine is zwitterionic, and its effect on the colloidal stability of AuNPs is most likely due to its association with citrate, affecting its charge state, resulting in reduction of the electrostatic stabilisation of the AuNPs. Using SERS as a solid-liquid interface probe provides a window into an interplay of interfacial and colloidal phenomena in the AuNP suspensions.

We report that gold-coated nanodisk arrays of nearly micron periodicity have high figure of merit (FOM) and sensitivity necessary for plasmonic refractometric sensing, with the added benefit of suitability for surface-enhanced Raman scattering (SERS), large scale microfabrication using standard photolithographic techniques and a simple instrumental setup. Gold nanodisk arrays were covered with a gold layer to excite the Bragg modes (BM) which are the propagative surface plasmons localized by the diffraction from the disk array. This generated surface-guided modes, localized as standing waves, leading to highly confined fields confirmed by a mapping of the SERS intensity and numerical simulations with 3D finite element method (3D FEM). The optimal gold-coated nanodisk arrays were applied for refractometric sensing in transmission spectroscopy with better performance than nanohole arrays and they were integrated to a 96-well plate reader for detection of IgY proteins in the nM range in PBS. The potential for sensing in biofluids was assessed with IgG detection in 1:1 diluted urine. The structure exhibits a high FOM of up to 46, exceeding the FOM of structures supporting surface plasmon polaritons (SPPs) and comparable to more complex nanostructures, demonstrating that sub-wavelength features are not necessary for high performance plasmonic sensing.

Surface-enhanced Raman scattering (SERS) is of interest for biomedical analysis and imaging because of its sensitivity, specificity and multiplexing capabilities. The successful application of SERS for in vivo biosensing requires probes to be biocompatible and procedures to be minimally invasive, challenges that have respectively been met by developing new nanoprobes and instrumentation. This Review presents recent developments in these areas, describing case studies in which sensors have been implemented, as well as outlining shortcomings that must be addressed before SERS sees clinical use. Surface-enhanced Raman scattering (SERS) is a physical phenomenon first discovered in 1974. SERS has since been exploited for bioanalysis because of its high sensitivity and multiplexing capabilities. This Review describes the progress made and problems faced with respect to using in vivo SERS in humans.

Artificial enzymes have become an increasingly interesting area of research due to their many advantages over natural protein enzymes which are expensive, difficult to isolate and unable to stand harsh environments. An important area of this research involves using metal nanoparticles as artificial enzymes, known as nanozymes, which exhibit peroxidase-like activity enabling them to catalyse the oxidation of substrates such as 3,3′,5,5′-tetramethylbenzidine (TMB) in the presence of hydrogen peroxide (H2O2), giving a colorimetric response. Here we exploit the catalytic activity of silver nanoparticles (Ag NPs) in a surface based silver-linked immunosorbent assay (SLISA) to detect human C-reactive protein (CRP), an inflammatory marker. Ag NPs were conjugated to antibodies with specific recognition for the corresponding target antigenic molecule, CRP, and the Ag NPs were used to catalyse the oxidation of TMB by H2O2. The resulting coloured oxidation product was detected using SERRS. We demonstrate that Ag NPs can replace the enzymes used in a conventional ELISA and a detection limit of 1.09 ng mL−1 of CRP can be achieved. It indicates the promise for SLISAs for biomarker detection and opens the way for further assays of this nature to be created. This novel assay has the potential to be optimised to detect lower levels of CRP and can be further extended for the sensitive and specific detection of other relevant biomarkers.

We report a novel approach for patterning thermoresponsive hydrogels based on N,N-diethylacrylamide (DEAAm) and bifunctional Jeffamine ED-600 by dip-pen nanolithography (DPN). The direct writing of micron-sized thermoresponsive polymer spots was achieved with efficient control over feature size. A Jeffamine-based ink prepared through the combination of organic polymers, such as DEAAm, in an inorganic silica network was used to print thermosensitive arrays on a thiol-silanised silicon oxide substrate. The use of a Jeffamine hydrogel, acting as a carrier matrix, allowed a reduction in the evaporation of ink molecules with high volatility, such as DEAAm, and facilitated the transfer of ink from tip to substrate. The thermoresponsive behaviour of polymer arrays which swell/de-swell in aqueous solution in response to a change in temperature was successfully characterised by atomic force microscopy (AFM) and Raman spectroscopy: a thermally-induced change in height and hydration state was observed, respectively. Finally, we demonstrate that cells can adhere to and interact with these dynamic features and exhibit a change in behaviour when cultured on the substrates above and below the transition temperature of the Jeffamine/DEAAm thermoresponsive hydrogels. This demonstrates the potential of these micropatterned hydrogels to act as a controllable surface for cell growth.