Meikun Fan

A review on the fabrication of substrates for surface enhanced Raman spectroscopy and their applications in analytical chemistry.

This work reviews different types of substrates used for surface-enhanced Raman scattering (SERS) that have been developed in the last 10 years. The different techniques of self-assembly to immobilize metallic nanoparticles on solid support are covered. An overview of SERS platforms developed using nanolithography methods, including electron-beam (e-beam) lithography and focused ion beam (FIB) milling are also included, together with several examples of template-based methodologies to generate metallic nano-patterns. The potential of SERS to impact several aspects of analytical chemistry is demonstrated by selected examples of applications in electrochemistry, biosensing, environmental analysis, and remote sensing. This review shows that highly enhancing SERS substrates with a high degree of reliability and reproducibility can now be fabricated at relative low cost, indicating that SERS may finally realize its full potential as a very sensitive tool for routine analytical applications.

Silver Nanoparticles on a Plastic Platform for Localized Surface Plasmon Resonance Biosensing

A cost-effective fabrication method for the preparation of localized surface plasmon resonance (LSPR) biosensors supported on plastics is described. The silver-nanoparticles-on-plastic sensor (SNOPS) was fabricated by chemically modifying the surface of a common plastic, polyethylene terephthalate (PET) to allow the efficient immobilization of Ag NPs. The LSPR of the SNOPS strip showed good sample-to-sample reproducibility. The analytical performance of the sensor strips for monitoring both thiol and protein adsorption, including bioaffinity, was examined. The limit of quantification to the adsorption of 11-mercaptoundecanoic acid was 500 nM and for the detection of streptavidin was approximately 9.5 nM. SNOPS can then be used as a cheap, versatile, and yet sensitive LSPR biosensor.

Surface-enhanced Raman scattering ( SERS ) from Au:Ag bimetallic nanoparticles : the effect of the molecular probe

Surface-enhanced Raman scattering (SERS) from molecular probes adsorbed on Au:Ag bimetallic nanoparticles with various compositions was investigated. Au:Ag bimetallic nanoparticles (NPs), with the diameters between 3–5 nm, were prepared and characterized by HRTEM and UV-Vis absorption. Their SERS properties were examined by using four different probe molecules, and compared with NPs made of pure Au or Ag. It is found that the SERS property of the alloy NPs is not only dependent on the Au:Ag ratio of the bimetallic NPs, but also on the chemical nature of the SERS probe. For the two positively charged SERS probes, oxazine 720 (Oxa) and Nile Blue A (NBA), the alloy NPs with higher Au content provided the largest SERS signal. However, for the probes 4-hydroxythiophenol (HTP) and thiophenol (TP), the best SERS performance was obtained for the highest Ag ratio. DFT calculations indicated a charge-transfer between Au and Ag atoms in the alloys, creating positively charged domains rich in Ag atom, and negatively charge regions dominated by Au atoms. It is proposed that the probe-specific enhancement is related to the selective binding of probe molecules to the partially charged surface domains in the alloys. Our results suggest that SERS substrate optimizations based on bimetallic nanoparticles should consider the nature of the probes and the electronic-induced effects from the alloys.

Fabrication of SERS swab for direct detection of trace explosives in fingerprints.

Swab sampling is of great importance in surface contamination analysis. A cotton swab (cotton Q-tip) was successfully transformed into surface-enhanced Raman scattering (SERS) substrate (SERS Q-tip) through a bottom-up strategy, where Ag NPs were first self-assembled onto the Q-tip followed by in situ growing. The capability for direct swab detection of Raman probe Nile Blue A (NBA) and a primary explosive marker 2,4-dinitrotoluene (2,4-DNT) using the SERS Q-tip was explored. It was found that at optimum conditions, a femotogram of NBA on glass surface could be swab-detected. The lowest detectable amount for 2,4-DNT is only ∼1.2 ng/cm(2) (total amount of 5 ng) on glass surface, 2 orders of magnitude more sensitive than similar surface analysis achieved with infrared technique, and comparable even with that obtained by ion mobility spectrometry-mass spectrometry. Finally, 2,4-DNT left on fingerprints was also analyzed. It was found that SERS signal of 2,4-DNT from 27th fingerprint after touching 2,4-DNT powder can still be clearly identified by swabbing with the SERS Q-tip. We believe this is the first direct SERS swabbing test of explosives on fingerprint on glass. Considering its relative long shelf life (>30 d), the SERS Q-tip may find great potential in future homeland security applications when combined with portable Raman spectrometers.

Self-Assembled Au Nanoparticles as Substrates for Surface-Enhanced Vibrational Spectroscopy: Optimization and Electrochemical Stability

Three-dimensional nanostructured metallic substrates for enhanced vibrational spectroscopy are fabricated by self-assembly. Nanostructures consisting of one to 20 depositions of 13 nm-diameter Au nanoparticles (NPs) on Au films are prepared and characterized by means of AFM and UV/Vis reflection-absorption spectroscopy. Surface-enhanced polarization modulation infrared reflection-absorption spectroscopy (PM-IRRAS) is observed from Au NPs modified by the probe molecule 4-hydroxythiophenol. The limitation of this kind of substrate for surface-enhanced PM-IRRAS is discussed. The surface-enhanced Raman scattering (SERS) from the same probe molecule is also observed and the effect of the number of Au-NP depositions on the SERS efficiency is studied. The SERS signal from the probe molecule maximizes after 11 Au-NP depositions, and the absolute SERS intensities from different batches are reproducible within 20%. In situ electrochemical SERS measurements show that these substrates are stable within the potential window between -800 and +200 mV (vs. Ag/AgCl/sat. Cl(-)).

Separation, identification and fast determination of organophosphate pesticide methidathion in tea leaves by thin layer chromatography–surface-enhanced Raman scattering

In this work, a fast, sensitive and convenient method for the on-site separation, identification and determination of organophosphate pesticide methidathion in tea leaves was reported, by the use of thin layer chromatography (TLC) in combination with surface-enhanced Raman spectroscopy (SERS). Five different organophosphate pesticides have been successfully separated and identified. Factors that affect the SERS detection sensitivity of methidathion, such as the brand of TLC plate, material and concentration of metallic nanoparticles (NPs), have been examined. It is found that the limit of quantification for methidathion is 0.1 ppm. Spiked tea samples of different kinds and brands containing methidathion at the ppm level have been tested, with recovery rates in the range of 86–113%. The proposed method for the fast on-site determination of pesticide may help to address food safety concerns in the general public.

Single point calibration for semi-quantitative screening based on an internal reference in thin layer chromatography-SERS: the case of Rhodamine B in chili oil

Thin layer chromatography (TLC) has been used in combination with surface enhanced Raman spectroscopy (SERS) for onsite screening of various analytes. In this work, we propose a novel concept for future field semi-quantitative SERS screening applications, where calibration curves were pre-built in the lab but subjected to onsite single point known standard amendment. Rhodamine B (RhB) in chili oil, a case of food scandal reported in China, was chosen as our model sample. A standard calibration curve of RhB was built using melamine as an internal standard and used throughout the assay. Before the analysis of samples, one single known RhB standard mixed with melamine was tested and used to calibrate the previously built standard calibration curve. RhB in chili oil was separated through the TLC method. Then, it was extracted and mixed with melamine. The signal of the mixture was recorded and compared with the single point calibrated calibration curve, instead of building a new curve each time. A limit of quantification (LOQ) of RhB from chili oil by SERS, 1.00 × 10−7 M, was realized for the first time, and the recovery range was from 66.1% to 110%, despite the fact that the cheapest and non-uniform SERS substrate was used. We expect such a protocol could be used for future fast onsite food quality assurance inspection.

3D printing of a mechanically durable superhydrophobic porous membrane for oil–water separation

Although superhydrophobic porous membranes are considered to be very promising candidates for oil–water separation, their fabrication methods often involve complicated treatments to build a coating with micro/nano-features on a porous mesh (called “coating on a mesh structure”), which can lead to weak mechanical stability of the superhydrophobic surfaces and the formation of inhomogeneous membrane pores. Herein, we report a facile and environmentally friendly 3D printing approach to fabricate superhydrophobic membranes with an ordered porous structure for oil–water separation using hydrophobic nanosilica-filled polydimethylsiloxane (PDMS) ink. The addition of nanosilica can improve the mechanical strength of the ink and thus ensures the formation of desired topographical structures without the risk of collapsing during 3D printing. Through adjusting the geometrical parameters, a superhydrophobic PDMS membrane was obtained, which mainly depended on the roughness at the sub-millimeter scale. More importantly, the 3D printing approach described herein integrated the superhydrophobic surface into the porous framework and resulted in a mechanically durable superhydrophobic membrane, which successfully avoids the weak interface adhesion issue that arises from the traditional “coating on a mesh structure.” Moreover, the pore size of the printed membrane could be easily adjusted via a computer program to optimize both the liquid flux and separation efficiency of the membranes. The maximum oil–water separation efficiency (∼99.6%) could be achieved for the printed porous membrane with the pore size of 0.37 mm, which also exhibited a high flux of ∼23 700 L m−2 h−1.

Surface enhanced Raman scattering fiber optic sensor as an ion selective optrode: the example of Cd2+ detection

Here in this work, we report the fabrication of a (metal) ion selective surface enhanced Raman scattering (SERS) optrode, the counterpart of an ion selective electrode, for the detection of metal ions in solution. Following our previous work, a layer-by-layer self-assembly strategy was used to fabricate the SERS optrode, followed by modification with an ion chelating reagent, 4-(4-phenylmethanethiol)-2,2′:6′,2′′-terpyridine (PMTTP). The SERS spectrum change after binding with metal ions was used to identify and detect metal ions in solution. Cd2+ in aqueous solution was chosen as a sample analyte. Similar to standard pH measurement, through simple single point known standard solution calibration, a quick (semi-)quantitative analysis of Cd2+ was realized.

Rapid and direct detection of illicit dyes on tainted fruit peel using a PVA hydrogel surface enhanced Raman scattering substrate

Food safety is one of the major concerns for consumers all around the world. Here in this work, we present a method that can be used for direct onsite fast screening of illicit additives on fruit peel. The method is based on our newly developed poly(vinyl alcohol) (PVA) hydrogel (slime) SERS substrate, which can conform to any surface shape. Simply by applying the hydrogel SERS substrate on the surface of interest, the limit of quantification for Sudan red (SR) III on a glass surface was found to be 1.6 ng/4 cm2. The time decay of SR III on a spiked kumquat was monitored with the proposed hydrogel SERS method and verified by HPLC. It was found that even after 25 days since dying, SR III could still be clearly identified at a level of dozens of ppb. With virtually no sample preparation requirement, the whole analysis procedure only took less than 5 min. Thus, the hydrogel SERS substrate based method could be used for future onsite food quality assurance applications when combined with a portable Raman spectrometer.