Zhengjun Gong

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.

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.

Facile preparation of silver nanoparticle decorated chitosan cryogels for point-of-use water disinfection

In this study, silver nanoparticle decorated chitosan (CS/Ag NP) cryogels were fabricated through a simple freeze-drying process for point-of-use (POU) water disinfection. The CS/Ag NP cryogels showed high porosity, good mechanical properties, an excellent water absorption capability, and most importantly, an efficient bactericidal feature. The absorption capacity for water was found to be 47g/g, approximately 90% of which was recovered by simple squeezing. Three different sizes of Ag NPs were compared regarding their bactericidal capability against both Escherichia coli (E. coli) and Bacillus subtilis (B. subtilis). Under optimum conditions, a 3 log reduction of bacteria was observed by holding the bacteria suspension (108 colony forming units (cfu)/mL) in the cryogels for 5min. Reduction was further increased to a 4 log when the contact time was doubled. The silver content in the cryogels was found to only be 7.5mg/g. Furthermore, the total Ag in processed water was found to only be 22μg/L, half of the safety limit set by China (<50μg/L). The bactericidal effectiveness of the material for real surface water samples was also demonstrated by treating water samples with different water quality matrices, including lake water and sewage water samples. In all three treated lake water samples, both the total bacteria and E. coli met the regulations for drinking water in China (<100cfu/mL for total bacteria and negative for E. coli). CS/Ag NP cryogels can be used for drinking water disinfection during disaster relief and in contingency water supply applications.

Killing Two Birds with One Stone: Coating Ag NPs Embedded Filter Paper with Chitosan for Better and Durable Point-of-Use Water Disinfection

In this study, porous chitosan (CS) coated Ag NPs embedded filter paper (CAEFP) was fabricated for point-of-use water disinfection application. Thanks for the presence of CS coating, the tensile strength of the CAEFP in wet condition was found to be 1.8 MPa, 700% increase compared with where there was no CS coating, making it much more durable. In addition, the coating with CS could greatly boost the Ag NPs loading without worrying about the excessive release of Ag into the treated water, thereby significantly improving the bactericidal efficiency but still be safe to drink in terms of Ag release. Furthermore, by controlling the amount of CS used, the flow rate and bactericidal efficiency of the CAEFP could be manipulated (customized). When the CS content increased from 0.5 to 2.0 wt %, the flow rate of CAEFP would drop from 9.3 to 0.53 L/min/m2, and the bactericidal efficiency against Escherichia coli and Bacillus subtilis could improve from 4 and 3.6 to 4.9 and 4.8 log reduction, respectively. At optimum condition, the total Ag in treated water by CAEFP was below 45 μg/L, only 1/10 of that from Ag NPs loaded filter paper without CS coating, half of the WHO drinking water requirement (<100 μg/L). Natural surface water samples were used for the demonstration of the bactericidal performance of the CAEFP. Both the total bacterial and E. coli counts met the WHO standard.