Ag nanocubes monolayer-modified PDMS as flexible SERS substrates for pesticides sensing

Helical au nanostructure for SERS detection of hazardous molecular and chiral enantiomers

In recent years, incidents of pesticide pollution and abuse of feed additives have occurred frequently, which pose a great threat to human health. Raman spectroscopy has become an important method in the field of food safety due to its rapidity, simplicity and sensitivity. It is important to obtain complex structure to promote surface-enhanced Raman scattering (SERS) effect. In this study, gold helical nanoparticles with rich surface structure were synthesized using cysteine as induce agent. Notably, the complex helical structure and tip led to an excellent electromagnetic enhancement property. The helical structure showed ultra-sensitive detection of hazardous molecular, such as thiram and ractopamine. Interestingly, the D/L-Au structure had significant chiral optical activity and could be used as an unlabeled SERS platform for enantiomer identification. This study provided an effective strategy for the detection of pesticides and feed additives, which could be applied in other aspects of food safety in the future.

SERS detection of thiram using a 3D sea cucumber-like composite flexible porous substrate

Nowadays, trace detection of thiram is in urgent demand due to its widespread application in agriculture and significant harmful effects on public health. In this work, a three-dimensional (3D) sea cucumber-like flexible porous surface-enhanced Raman scattering (SERS) substrate composed of a poly(vinylidene fluoride) (PVDF) membrane, ZnO nanorods, gold films, and Ag nanoparticles (Ag/Au/ZnO/P) has been established for the highly sensitive detection of thiram. The substrate takes advantage of the 3D morphology of the Ag/Au/ZnO system on a flexible porous PVDF membrane to produce abundant plasmonic hot spots. Meanwhile, the employment of an AgNPs/Au shell system combined the benefits of both gold and silver metals, thus guaranteeing stable and sensitive detection. With 4-mercaptobenzoic acid (4-MBA) as a probe molecule, the Ag/Au/ZnO/P substrate exhibited excellent linear detection in the range of 10-11-10-5 M, with a correlation coefficient (R2) of 0.99 and an enhancement cofactor of 7.09 × 107. The substrate exhibited excellent uniformity with a related standard deviation (RSD) value of 3.82% and demonstrated high stability during a 15 d-storage test. In addition, the substrate could detect thiram in an aqueous solution at concentrations as low as 10-10 M with excellent selectivity. Meanwhile, thiram on the surface of apple peel could be easily detected by the Ag/Au/ZnO/P substrate with the "paste-and-peel" method in less than 10 s, and the detection limit could be as low as 0.48 ng cm-2. Overall, the remarkable performance of the Ag/Au/ZnO/P SERS substrate demonstrated its great potential for the environmental monitoring of thiram.

Laser polarization as a critical factor in the SERS-based molecular sensing performance of nano-gapped Au nanowires

Nowadays, Au dimer-based nanostructures are exhaustively studied due to their outstanding potential as plasmonic nanoantennas for future applications in high-sensitivity molecular sensing by Surface-Enhanced Raman Spectroscopy (SERS). In this work, we analyze nano-gapped Au nanowires (NWs) or Au-NW dimers for designing efficient nanoantennas, reporting an exhaustive study about dimer length and laser polarization orientation effects on their SERS-based molecular sensing performance. Arrays of nanoantennas with gaps of about 22 ± 4 nm, nominal square cross-sections of 60 nm × 60 nm, and different segment lengths from 300 nm up to 1200 nm were fabricated by Au evaporation and subsequent e-beam lithography. The SERS performance was studied by confocal Raman microscopy using a linearly-polarized 633 nm laser. A critical impact of the polarization alignment on the spectral resolution of the studied Raman marker imprint was observed. The results show that the Raman signal is maximized by aligning the polarization orientation with the nanowire long axis, it is reduced by increasing the relative angle, and it is abruptly minimized when both are perpendicular. These observations were consistent with numerical simulations carried out by the FDTD method, which predicts a similar dependence between the orientation of linearly-polarized light and electric-near field amplification in the nano-gap zone. Our results provide an interesting paradigm and relevant insights in determining the role of laser polarization in the Raman signal enhancement in nano-gapped Au nanowires, showing the key role of this measurement condition on the SERS-based molecular sensing efficiency of this kind of nanostructure.

High-performance, large-area flexible SERS substrates prepared by reactive ion etching for molecular detection

In the realm of molecular detection, the surface-enhanced Raman scattering (SERS) technique has garnered increasing attention due to its rapid detection, high sensitivity, and non-destructive characteristics. However, conventional rigid SERS substrates are either costly to fabricate and challenging to prepare over a large area, or they exhibit poor uniformity and repeatability, making them unsuitable for inspecting curved object surfaces. In this work, we present a flexible SERS substrate with high sensitivity as well as good uniformity and repeatability. First, the flexible polydimethylsiloxane (PDMS) substrate is manually formulated and cured. SiO2/Ag layer on the substrate can be obtained in a single process by using ion beam sputtering. Then, reactive ion etching is used to etch the upper SiO2layer of the film, which directly leads to the desired densely packed nanostructure. Finally, a layer of precious metal is deposited on the densely packed nanostructure by thermal evaporation. In our proposed system, the densely packed nanostructure obtained by etching the SiO2layer directly determines the SERS ability of the substrate. The bottom layer of silver mirror can reflect the penetrative incident light, the spacer layer of SiO2and the top layer of silver thin film can further localize the light in the system, which can realize the excellent absorption of Raman laser light, thus enhancing SERS ability. In the tests, the prepared substrates show excellent SERS performance in detecting crystalline violet with a detection limit of 10-11M. The development of this SERS substrate is anticipated to offer a highly effective and convenient method for molecular substance detection.

Cotton swabs wrapped with three-dimensional silver nanoflowers as SERS substrates for the determination of food colorant carmine on irregular surfaces

A lightweight, portable, low-cost, and accessible cotton swab was employed as surface enhanced Raman spectroscopy (SERS) matrix template. The silver nanoflowers were in situ grown on the surface of cotton swabs to form three-dimensional Ag nanoflower@cotton swabs (AgNF@CS) SERS substrate with high-density and multi-level hot spots. The SERS performance of AgNFs@CS substrates with various reaction time was systematically studied. The optimal AgNF-120@CS SERS substrate exhibits superior detection sensitivity of 10-10 M for methylene blue, good signal reproducibility, high enhancement factor of 1.4 × 107, and excellent storage stability (over 30 days). Moreover, the AgNF-120@CS SERS substrate also exhibits prominent detection sensitivity of 10-8 M for food colorant of carmine. Besides, the portable AgNF-120@CS SERS substrate is also capable of detecting food colorant residues on irregular food surfaces.

A core-shell structured AuNPs@ZnCo-MOF SERS substrate for sensitive and selective detection of thiram

Surface-enhanced Raman scattering (SERS) enables pesticide residue monitoring to become facile and efficient. In this study, a core-shell structured gold nanoparticles@ZnCo metal-organic framework (AuNPs@ZnCo-MOF) SERS substrate was designed and successfully synthesized for efficient and selective detection of thiram. The bimetallic ZnCo-MOF shell can not only enrich the targeted molecules in the electromagnetic field because of its excellent absorptive capacity, but also act as a stabilized matrix for protecting the AuNPs from aggregation. The AuNPs@ZnCo-MOFs exhibited a high enhancement factor (EF) of 3.51 × 106 and a low detection limit of 1 × 10-7 mol L-1. Besides, the substrate material showed exceptional stability for up to 28 days at room temperature. The AuNPs@ZnCo-MOFs were used to detect thiram which displayed wide linearity (1 × 10-7 to 1 × 10-4 mol L-1) and high recoveries (83.45-99.61%). Moreover, the AuNPs@ZnCo-MOF SERS substrate exhibited excellent anti-interference ability and size selectivity for the target molecules. These indicate that the AuNPs@ZnCo-MOF substrate has great potential for the detection of thiram residues in practical applications.

Inner filter effect-based upconversion nanosensor for rapid detection of thiram pesticides using upconversion nanoparticles and dithizone-cadmium complexes

Herein, we propose a method for detecting thiram based on the fluorescence inner filter effect using upconversion nanoparticles and dithizone-cadmium complexes (DZ-Cd2+). The ultraviolet absorption of DZ-Cd2+ was in the range of 480-600 nm under alkaline conditions, resulting in fluorescence quenching of the nanoparticles at 540 nm. Thiram had a stronger coordination effect with Cd2+ than dithizone; thus, more thiram-cadmium complex (T-Cd2+) formed when thiram was added, leading to fluorescence recovery at 540 nm. The standard thiram curve was found to have a detection limit of 6.75 ng/mL in the linear range of 0.01-1000 µg/mL. In addition, high-performance liquid chromatography results for detecting thiram in apple samples revealed good application performance. The results demonstrate that the developed method has great potential to detect thiram residues in food.

AgTNP@TiO2@Ag core-satellite composites for sensitive sensing and in situ monitoring photodegradation of organic dyes by portable Raman spectrometer

The development of sensitive, reliable, and robust substrates for surface-enhanced Raman spectroscopy (SERS) heavily relies on the creation of numerous hot spots. In this study, we propose a simple approach to fabricate core-satellite composites composed of AgTNP@TiO2@Ag, where Ag triangular nanoplates (AgTNPs) act as the cores, TiO2 serves as the interlayer, and Ag nanoparticles are deposited around them to form Ag satellites. By adjusting the amount of AgNO3, we precisely control the coverage of Ag nanoparticles on AgTNP@TiO2@Ag, thus fine-tuning their SERS sensitivity. Various characterization techniques were employed to examine their composition, morphology, and crystal structure. Thanks to the abundant hot spots created by the Ag satellites, these composites exhibit significantly enhanced SERS sensitivity and they demonstrate the capability to detect methylene blue (MB) at a concentration of 10-10 M by portable Raman spectrometer. Moreover, the AgTNP@TiO2@Ag composites effectively enable in situ SERS monitoring of the photodegradation reaction of MB. Overall, the novel AgTNP@TiO2@Ag composites prepared in this study exhibit high SERS sensitivity and excellent photocatalytic performance, making them highly valuable for environmental detection and ecological restoration purposes.

Self-assembly flexible SERS imprinted membrane based on Ag nanocubes for selective detection of microcystin-LR

Silver nanocubes monolayer-modified polydimethylsiloxane (Ag NC/PDMS) flexible SERS substrates have been prepared by a three-phase interface self-assembly procedure. The combination of this method with membrane technology brings nanoparticles in close proximity, densely, and regularly arranged in monolayers over a large area, leading to excellent SERS properties. Considering the complexity of practical detection, molecular imprinted polymers (MIPs) were anchored on the surface of SERS substrate and applied to selective detection of microcystin-LR (MC-LR). It is worth mentioning that the SERS imprinted membranes (AP-MIMs) were still clearly detected at a concentration of 0.1 µg·L-1 of MC-LR in drinking water, and the detection limit was as low as 0.0067 µg·L-1. The substrate exhibited excellent uniformity with a relative standard deviation (RSD) of 6.1%. In the presence of interference molecules, AP-MIMs exhibited excellent selectivity for MC-LR. Furthermore, in the spiking and recovery tests of practical lake water samples, the method showed excellent recoveries ranging from 96.47 to 105.31%. It has been demonstrated that the prepared AP-MIMs can be applied to sensitive and specific detection of trace amounts of MC-LR in drinking water.

Modified paper-based substrates fabricated via electrostatic attraction of gold nanospheres for non-destructive detection of pesticides based on surface-enhanced Raman spectroscopy

BACKGROUND

Flexible surface-enhanced Raman spectroscopy (SERS) substrates such as paper-based substrates show great potential for rapid detection of residual chemicals on food surfaces. However, controlling the density and distribution of metallic nanoparticles adsorbed on the paper is still challenging.

RESULTS

The amount of gold (Au) nanospheres (51 ± 4 nm) attached on the filter paper modified with 3-aminopropyltriethoxysilane (APTES) was tunable, increasing as the level of APTES (2.5-15.0 g kg-1 ) applied for paper modification increased. Moreover, the Au nanospheres were relative evenly distributed on the filter paper modified with 2.5-10.0 g kg-1 of APTES, which resulted in excellent intra- and inter-reproducibility of SERS signals for pesticides including thiram, diquat dibromide, and paraquat dichloride (relative standard deviation = 2.2-10.1%). The modified paper-based substrate could be used to detect as low as 0.05-0.2 mg L-1 of pesticides in standard solutions, and as low as 5-20 ng cm-2 of residual pesticides on apple skins with minimum sample pretreatment.

CONCLUSION

This paper-based substrate with tunable feature for the density and distribution of nanoparticles is applicable for rapid SERS detection of residual pesticides in fruits and vegetables. © 2023 Society of Chemical Industry.

Recent Advances in Aptasensors for Rapid Pesticide Residues Detection

Pesticides are applied widely to increase agricultural output and quality, however, this practice results in residual issues that not only harm the environment but also put people and animals' lives and health at risk. As a result, it is critical to find pesticide residues in a variety of sources, including crops, water supplies, and soil. Aptamers are more flexible in their synthesis and modification, have a high level of specificity, are inexpensive, and have good stability compared to conventional detection methods. They have therefore attracted a lot of interest in the industry. This study reviews the most recent aptasensor advancements in the detection of pesticide residues. Firstly, aptamers specifically binding to many pesticides are summarized. Secondly, the combination of aptasensors with colorimetric, fluorescent, surface enhanced Raman spectroscopy (SERS), resonance Light Scattering (RLS), chemiluminescence (CL), electrochemical, and electrochemiluminescence (ECL) technologies are systematically introduced, and their advantages and disadvantages are expounded. Importantly, the aptasensors for the detection of various pesticides (organochlorine, organophosphorus, neonicotinoids, carbamates, and pyrethroids) that have been developed so far are systematically analyzed and discussed. Finally, the furture prospects and challenges of the aptasensors are highlighted. It is expected to offer suggestions for the later creation of novel, highly effective and sensitive aptasensors for the detection of pesticide residues.

Casting liquid PDMS on self-assembled bilayer polystyrene nanospheres to prepare a SERS substrate with two layers of nanopits for detection of p-nitrophenol

p-Nitrophenol (PNP) is widely used in pesticides, pharmaceuticals, and dyestuffs. It is vital to detect trace PNP in the environment, because it poses significant environmental hazards due to its high toxicity. In this paper, a new method was reported for preparing a SERS substrate with excellent SERS activity by combining self-assembly techniques and flexible materials. First, the three-dimensional (3D) polystyrene (PS) photonic crystal (PC) structural master was fabricated by stacking two layers of self-assembled PS nanospheres with different diameters. Polydimethylsiloxane (PDMS) with a complementary structure to the master was obtained by casting, curing and peeling off. Finally, the PDMS-Ag substrate was fabricated by sputtering a thin Ag layer on the PDMS structure. The enhancement factor (EF) of the PDMS-Ag substrate was calculated to be 2.90 × 109 by using 4-amino thiophenol (ATP) as the probe molecule, and the limit of detection (LOD) for ATP can reach 10-11 M. And the RSD of the SERS intensity for the peak at 1078 cm-1 on the PDMS-Ag substrates from batch to batch was within 2%, indicating the high reproducibility of the as-prepared substrate. The quantitative analysis of PNP was achieved with a LOD of 10-8 M. Therefore, the PDMS-Ag substrate exhibits high sensitivity and reproducibility, and it can detect PNP in trace amounts, with great potential for detecting other contaminants.

Ratiometric SERS-based assay with "sandwich" structure for detection of serotonin

A ratiometric nanoensemble-functionalized Surface-Enhanced Raman Spectroscopy (SERS) chip is proposed and an ultrasensitive "sandwich" structure introduced for the detection of 5-HT to achieve early diagnosis of colon cancer. The SERS-based chip contains core-shell SERS active substrates coded by different Raman tags with Raman-silent region peaks (Au@EBP@Au NR arrays and Au@MBN@Ag NPs) and then identify-function molecule modification to construct the "sandwich" structure (Au@EBP@Au NR arrays/5-HT/Au@MBN@Ag NPs). Au@EBP@Au NR arrays showed excellent SERS performance, including good uniformity with an RSD of 5.53% and an enhancement factor (EF) of 2.13 × 10⁷. The intensity ratio of the peaks in the Raman silent region was proportional to the concentration of 5-HT in the range 5 × 10^-7-1 × 10^-3 M, with a detection limit (LOD) of 4.9 × 10^-9 M. Excellent assay accuracy was also demonstrated, with recoveries in the range 96.80% to 104.96%. Finally, we found that 5-HT expression levels in normal human sera were much lower than those in colon cancer patients by using a SERS-based chip for determination of the concentration of 5-HT in clinical colon cancer serum. This result suggested that the proposed approach has potential for detecting 5-HT by ratiometric SERS-based chips for early diagnosis of colon cancer.

Engineering an Ag/Au bimetallic nanoparticle-based acetylcholinesterase SERS biosensor for in situ sensitive detection of organophosphorus pesticide residues in food

Developing simple, efficient, and inexpensive method for trace amount organophosphorus pesticides' (OPs) detection with high sensitivity and specificity is of significant importance for guaranteeing food safety. Herein, an Ag/Au bimetallic nanoparticle-based acetylcholinesterase (AChE) surface-enhanced Raman scattering (SERS) biosensor was constructed for in situ simple and sensitive detection of pesticide residues in food. The principle of this biosensor exploited 4-mercaptophenylboronic acid (4-MPBA)-modified Ag/Au bimetallic nanoprobes as SERS signal probe to improve sensitivity and stability. The combination of AChE and choline oxidase (CHO) can hydrolyze acetylcholine (ATCh) to generate H₂O₂. The product of H₂O₂ selectively oxidizes the boronate ester of 4-MPBA, decreasing the Raman intensity of the B-O symmetric stretching. In the presence of OPs, it could inhibit the production of H₂O₂ by destroying the AChE activity, so the reduction of the SERS signal was also alleviated. Based on the principle, an Ag/Au bimetallic nanoparticle-based AChE SERS sensor was established without any complicated pretreatments. Benefiting from the synergistic effects of Ag/Au bimetallic hybrids, a linear detection range from 5×10^-9 to 5×10^-4 M was achieved with a limit of detection down to 1.7×10^-9 M using parathion-methyl (PM) as the representative model of OPs. Moreover, the SERS biosensor uses readily available reagents and is simple to implement. Importantly, the proposed SERS biosensor was used to quantitatively analyze OP residues in apple peels. The levels of OPs detected in real samples by this method were consistent with those obtained using gas chromatography-mass spectrometry (GC-MS), suggesting the proposed assay has great potential applications for OPs in situ detection in food safety fields.

Hollow and nanoporous Ag sub-microcubes as SERS substrates

In this work, Ag(NH₃)₂NO₃ microcubes are obtained by directly evaporating the silver ammonia complex solution. In Na₃Cit solution, each Ag(NH₃)₂NO₃ microcube can crumble into lots of Ag₃Cit sub-microcubes with uniform size, which serve as sacrificial templates to yield Ag sub-microcages. The as-obtained Ag cages possess ultra-high surface-enhanced Raman scattering (SERS) sensitivity towards R6G and thiram.

Rapid Detection of Malathion, Phoxim and Thiram on Orange Surfaces Using Ag Nanoparticle Modified PDMS as Surface-Enhanced Raman Spectroscopy Substrate

Malathion, phoxim, and thiram are organophosphates and organosulfur pesticides widely used in agricultural products. The residues of these pesticides present a direct threat to human health. Rapid and on-site detection is critical for minimizing such risks. In this work, a simple approach was introduced using a flexible surface-enhanced Raman spectroscopy (SERS) substrate. The prepared Ag nanoparticles-polydimethylsiloxane (AgNPs-PDMS) substrate showed high SERS activity, good precision (relative standard deviation = 5.33%), and stability (30 days) after optimization. For target pesticides, the linear relationship between characteristic SERS bands and concentrations were achieved in the range of 10~1000, 100~5000, and 50~5000 μg L-1 with LODs down to 3.62, 41.46, and 15.69 μg L-1 for thiram, malathion, and phoxim, respectively. Moreover, SERS spectra of mixed samples indicated that three pesticides can be identified simultaneously, with recovery rates between 96.5 ± 3.3% and 118.9 ± 2.4%, thus providing an ideal platform for detecting more than one target. Pesticide residues on orange surfaces can be simply determined through swabbing with the flexible substrate before acquiring the SERS signal. This study demonstrated that the prepared substrate can be used for the rapid detection of pesticides on real samples. Overall, this method greatly simplified the pre-treatment procedure, thus serving as a promising analytical tool for rapid and nondestructive screening of malathion, phoxim, and thiram on various agricultural products.