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Rapid and quantitative detection of malachite green (MG) in aquaculture products is very important for safety assurance in food supply. Here, we develop a point-of-care testing (POCT) platform that combines a flexible and transparent surface-enhanced Raman scattering (SERS) substrate with deep learning network for achieving rapid and quantitative detection of MG in fish. The flexible and transparent SERS substrate was prepared by depositing silver (Ag) film on the polydimethylsiloxane (PDMS) film using laser molecular beam epitaxy (LMBE) technique. The wrinkled Ag NPs@PDMS film exhibits high SERS activity, excellent reproducibility and good mechanical stability. Additionally, the fast in situ detection of MG residues onfishscales was achieved by using the wrinkled Ag NPs/PDMS film and a portable Raman spectrometer, with a minimum detectable concentration of 10-6 M. Subsequently, a one-dimensional convolutional neural network (1D CNN) model was constructed for rapid quantification of MG concentration. The results demonstrated that the 1D CNN quantitative analysis model possessed superior predictive performance, with a coefficient of determination (R2) of 0.9947 and a mean squared error (MSE) of 0.0104. The proposed POCT platform, integrating a transparent flexible SERS substrate, a portable Raman spectrometer and a 1D CNN model, provides an efficient strategy for rapid identification and quantitative analysis of MG in fish.

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Flexible and transparent surface-enhanced Raman scattering (SERS) substrates have attracted considerable attention for their ability to enable the direct in situ detection of analytes on curved surfaces. However, the curvature of an object can impact the signal enhancement of SERS during the measurement process. Herein, we propose a simple approach for fabricating a curvature-insensitive transparent SERS substrate by depositing silver nanoparticles (Ag NPs) onto a large-area wrinkled polystyrene/polydimethylsiloxane (Ag NP@W-PS/PDMS) bilayer film. Using rhodamine 6G (R6G) as a probe molecule, the optimized Ag NP@W-PS/PDMS film demonstrates a high analytical enhancement factor (AEF) of 4.83 × 105, excellent uniformity (RSD = 7.85%) and reproducibility (RSD = 3.09%), as well as superior mechanical flexibility. Additionally, in situ measurements of malachite green (MG) on objects with diverse curvatures, including fish, apple, and blueberry, are conducted using a portable Raman system, revealing a consistent SERS enhancement. Furthermore, a robust linear relationship (R2 ≥ 0.990) between Raman intensity and the logarithmic concentration of MG detected from these objects is achieved. These results demonstrate the tremendous potential of the developed curvature-insensitive SERS substrate as a point-of-care testing (POCT) platform for identifying analytes on irregular objects.

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Flexible and transparent surface-enhanced Raman scattering (SERS) substrates haveattractedmuchattention as a fast, sensitive and in situ detection platform for practical applications. However, the large-area fabrication of flexible and transparent SERS substrates with high performance is still challenging. Here, a flexible and transparent SERS substrate based on large-area thin PDMS film decorated with Ag microlabyrinth/nanoparticles hierarchical structures (denoted as ALNHS@PDMS) is fabricated by using the floating-on-water method and magnetron sputtering technology. By optimizing the sputtering time, the ALNHS with multiple hot spots are uniformly distributed on the PDMS surface. Based on characterizing the rhodamine 6G (R6G) with a portable Raman spectrometer, the optimal ALNHS@PDMS film exhibits a high enhancement factor (5.2 × 106), excellent uniformity and reproducibility, as well as superior mechanical stability. In addition, thanks to the good sticky feature and bi-directional activation property of the thin ALNHS@PDMS film, the prepared flexible and transparent SERS substrate can achieve in situ detection of malachite green residues (10-6 M) on apple and tomato skins. This large-area, thin, mechanically robust, flexible and transparent ALNHS@PDMS film, integrated with a portable Raman spectrometer, shows great potential for point-of-care testing (POCT)in practical applications.

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Flexible surface-enhanced Raman spectroscopy (SERS) substrate has attracted great attention due to its convenient sampling and on-site monitoring capability. However, it is still challenging to fabricate a versatile flexible SERS substrate, which can be used for in situ detection of analytes either in water or on irregular solid surfaces. Here, we report a flexible and transparent SERS substrate based on a wrinkled polydimethylsiloxane (PDMS) film obtained by transferring corrugated structures on the aluminium/polystyrene bilayer film, onto which silver nanoparticles (Ag NPs) are deposited by thermal evaporation. The as-fabricated SERS substrate exhibits a high enhancement factor (∼1.19×105), good signal uniformity (RSD of 6.27%), and excellent batch-to-batch reproducibility (RSD of 7.3%) for rhodamine 6 G. In addition, the Ag NPs@W-PDMS film can maintain high detection sensitivity even after mechanical deformations of bending or torsion for 100 cycles. More importantly, being flexible, transparent, and light, the Ag NPs@W-PDMS film can both float on the water surface and conformally contact with the curved surface for in situ detection. The malachite green in aqueous environment and on apple peel can be easily detected down to 10-6 M with a portable Raman spectrometer. Therefore, it is expected that such a versatile flexible SERS substrate has great potential in on-site, in situ contaminant monitoring for realistic applications.

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The large-area fabrication of flexible and transparent surface-enhanced Raman scattering (SERS) substrates with high performance by a facile and efficient method is still challenging. Here, we demonstrated a large-scale, flexible and transparent SERS substrate composed of PDMS nanoripple array film decorated with silver nanoparticles (Ag NPs@PDMS-NR array film) prepared by a combination of plasma treatment and magnetron sputtering. The performances of SERS substrates were characterized by rhodamine 6G (R6G) using a handheld Raman spectrometer. The optimal Ag NPs@PDMS-NR array film exhibited high SERS sensitivity, with a detection limitation of R6G reaching 8.20 × 10-8 M as well as excellent uniformity (RSD = 6.8%) and batch-to-batch reproducibility (RSD = 2.3%). In addition, the substrate showed outstanding mechanical stability and good SERS enhancement by backside illumination, thus it was suitable for in situ SERS detection on curved surfaces. The detection limit of malachite green on apple and tomato peels was 1.19 × 10-7 and 1.16 × 10-7 M, respectively, and quantitative analysis of pesticide residues could be realized. These results demonstrate that the Ag NPs@PDMS-NR array film has great practical potential in rapid in situ detection of pollutants.

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With the development of flexible surface-enhanced Raman spectroscopy (SERS) substrates that can realize rapid in situ detection, the SERS technique accompanied by miniaturized Raman spectrometers holds great promise for point-of-care testing (POCT). For an in situ detection strategy, constructing high-performance flexible and transparent SERS substrates through a facile and cost-effective fabrication method is critically important. Herein, we present a simple method for fabricating a large-area flexible and transparent SERS substrate consisting of a silver-nanoparticle-grafted wrinkled polydimethylsiloxane (Ag NPs@W-PDMS) film, using a surface-wrinkling technique and magnetron sputtering technology. By characterizing rhodamine 6G as a probe molecule with a portable Raman spectrometer, the flexible SERS substrate shows a low detection limit (10-7 M), a high enhancement factor (6.11 × 106), and excellent spot-spot and batch-batch reproducibilities (9.0% and 4.2%, respectively). Moreover, the Ag NPs@W-PDMS substrate maintains high SERS activity under bending and twisting mechanical deformations of over 100 cycles, as well as storage in air for 30 days. To evaluate its practical feasibility, in situ detection of malachite green on apple and tomato peels is performed with a detection limit of 10-6 M. In addition, for point-of-care analysis, we develop a wireless transmission system to transmit the collected SERS spectral data to a computer in real time for signal processing and analysis. Therefore, the proposed Ag NPs@W-PDMS SERS substrate fabricated through a simple and mass-producible method, combined with the utilization of a portable Raman spectrometer and wireless communication, offers a promising opportunity to extend the SERS technique from the laboratory to POCT applications.

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DBP, one of the phthalic acid esters (PAEs), is known as an endocrine disruptor and is toxic to humans in abnormal concentrations. Here, a high-density and ordered SERS substrate based on the self-assembly of triangular Ag nanoplate (TAgNP) arrays is developed for DBP detection. Benefiting from the ordered arrangement and sharp corners of TAgNPS, the arrays can provide sufficient and uniform hotspots for reproducible and highly active SERS effects. Using Rhodamine 6G (R6G) as a reporter molecule, the SERS enhancement factor (EF) of the TAgNP arrays was found to be as high as 1.2 × 107 and the relative standard deviation was 6.56%. As a trial for practical applications, the TAgNP array substrates were used for the detection of dibutyl phthalate (DBP) in edible oils. In this assay, edible oil samples were added to hexane as an organic phase for the formation of the TAgNP arrays, which caused DBP to be loaded at hotspots. DBP in edible oils could be identified at concentrations as low as 10-7 M. This SERS substrate based on the TAgNP arrays has great potential applications in the high sensitivity and reproducible detection of contaminants in food.

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A two-component gas sensor in quartz-enhanced photoacoustic spectroscopy based on time-division multiplexing (TDM) technology of a distributed-feedback (DFB) laser driver current was proposed and experimentally demonstrated. The quartz tuning-fork-based photoacoustic spectroscopy (PAS) cell configuration with two optical collimators and two acoustic microresonators was designed to detect the second-harmonic (${2}f$2f) PAS signal. The two optical collimators guaranteed that the two laser beams would inject the PAS cell conveniently, providing higher power input than a 3 dB optical fiber coupler. Two-component gas sensing was achieved by the TDM of the DFB laser driver current. We used this two-component gas sensing technique to detect acetylene (${{\rm C}_2}{{\rm H}_2}$C2H2) at 1532.83 nm and methane (${{\rm CH}_4}$CH4) at 1653.722 nm. The ${{\rm C}_2}{{\rm H}_2}$C2H2 and ${{\rm CH}_4}$CH4 detection was achieved at a 2.4 s interval. The minimum detection limits of 1 ppmv for ${{\rm C}_2}{{\rm H}_2}$C2H2 and 13.14 ppmv for ${{\rm CH}_4}$CH4 were obtained, and the linear responses reached were 0.99968 and 0.99652 for ${{\rm C}_2}{{\rm H}_2}$C2H2 and ${{\rm CH}_4}$CH4, respectively. Moreover, the continuous monitoring of ${{\rm CH}_4}$CH4 and ${{\rm C}_2}{{\rm H}_2}$C2H2 for 40 min showed a good stability. The TDM technology of the DFB laser driver current would play an important role on the multi-component detection.

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In this paper, acquisition of the valence Compton profile of few-layer graphene using electron energy-loss spectroscopy at large scattering angle is reported. The experimental Compton profile is compared with the corresponding theoretical profile, calculated using the full-potential linearized augmented plane wave method based on the local-density approximation. Good agreement exists between the theoretical calculation and experiment. The graphene profile indicates a substantially greater delocalization of the ground state charge density compared to that of graphite.

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Plasmonic nanomaterials possessing large-volume, high-density hot spots with high field enhancement are highly desirable for ultrasensitive surface-enhanced Raman scattering (SERS) sensing. However, many as-prepared plasmonic nanomaterials are limited in available dense hot spots and in sample size, which greatly hinder their wide applications in SERS devices. Here, we develop a two-step physical deposition protocol and successfully fabricate 3D hierarchical nanostructures with highly dense hot spots across a large scale (6 × 6 cm2 ). The nanopatterned aluminum film was first prepared by thermal evaporation process, which can provide 3D quasi-periodic cloud-like nanostructure arrays suitable for noble metal deposition; then a large number of silver nanoparticles with controllable shape and size were decorated onto the alumina layer surfaces by laser molecular beam epitaxy, which can realize large-area accessible dense hot spots. The optimized 3D-structured SERS substrate exhibits high-quality detection performance with excellent reproducibility (13.1 and 17.1%), whose LOD of rhodamine 6G molecules was 10-9 M. Furthermore, the as-prepared 3D aluminum/silver SERS substrate was applied in detection of melamine with the concentration down to 10-7 M and direct detection of melamine in infant formula solution with the concentration as low 10 mg/L. Such method to realize large-area hierarchical nanostructures can greatly simplify the fabrication procedure for 3D SERS platforms, and should be of technological significance in mass production of SERS-based sensors.

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Valence Compton profiles (CPs) of multiwall (MWCNTs) and single-wall carbon nanotubes (SWCNTs) were obtained by recording electron energy-loss spectra at large momentum transfer in the transmission electron microscope, a technique known as electron Compton scattering from solids (ECOSS). The experimental MWCNT/SWCNT results were compared with that of graphite. Differences between the valence CPs of MWCNTs and SWCNTs were observed, and the SWCNT CPs indicate a greater delocalization of the ground-state charge density compared to graphite. The results clearly demonstrate the feasibility and potential of the ECOSS technique as a complementary tool for studying the electronic structure of materials with nanoscale spatial resolution.

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Organic-inorganic hybrid heterojunctions are ever so promising for low-cost and high-efficient photoelectric devices. We report a p-n junction diode composed of inorganic ZnO nanowire arrays (ZNAs) and organic 2, 2', 7, 7'-tetrakis-(N, N-di-p-methoxyphenylamine)- 9, 9'-spirobifluorene (spiro-MeOTAD). A maximum photoresponsivity of 1.32 mA/W is observed under illumination at zero bias. We also demonstrate that the photocurrent time response is observed to be rapid, consistent, as well as repeatable. On blue light weak irradiance (410 nm, 75 µW/cm2), the rise time constant and decay time constant were found to be 0.12 and 0.06 s, respectively. These results suggest ZNAs/spiro-MeOTAD heterojunction as a candidate for an efficient self-powered blue light detector.

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Three-dimensional (3D) plasmonic structures have been intensively investigated as high performance surface enhanced Raman scattering (SERS) substrates. Here, we demonstrate a 3D biomimetic SERS substrate prepared by deposition of silver nanoparticles (Ag NPs) on the bioscaffold arrays of cicada wings using laser molecular beam epitaxy. This deposition method can offer a large number of nanoparticles with average diameter of ∼10 nm and nanogaps of sub-10 nm on the surface of chitin nanopillars to generate a high density of hotspots. The prepared 3D Ag/cicada SERS substrate shows a limit of detection (LOD) for Rhodamine 6G as low as 10-7 M, high enhancement factor of 1.09 × 105, and excellent signal uniformity of 6.8%. Moreover, the molecular fingerprints of melamine in infant formula can be directly extracted with an LOD as low as 10 mg L-1, without the need for functional modification. The prepared SERS-active substrate, due to its low cost, high-throughput, and good detection performance, can be widely used in applications such as food safety and environmental monitoring.

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This paper reports the numerical study of the one-step faithful replication of micro/nano-scale structures on a fiber surface by using the electrohydrodynamic instability patterning (EHDIP) process. By employing a rigorous numerical analysis method, conditions are revealed under which the faithful replication of a pattern can be achieved from a curved master electrode. It is found that the radius of curvature of the fiber plays an important role in determining the final morphology of the pattern when the destabilizing electric field is dominant in both the flat and patterned template cases. In general, stronger electric fields and larger radii of curvature of the substrate are favorable for the faithful replication of the pattern. In addition, theoretical analysis shows that higher aspect ratio of micro/nanostructures can be obtained on curved surfaces by using a master with a much lower aspect ratio. The results demonstrated in this study aims to provide guidelines for the faithful fabrication of micro/nanostructures on curved surfaces by the EHDIP process.

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