Synthesis of novel gold nanostars as surface-enhanced raman spectroscopy substrate for measurement of pesticide residues in fresh produce
[ACCESS RESTRICTED TO THE UNIVERSITY OF MISSOURI AT REQUEST OF AUTHOR.] Rapid detection of food contaminants using novel analytical methods in combination with nanomaterials has received much attention in recent years. This study aimed to synthesize polyhedral gold nanostars (AuNS) with multi-angled corners and develop surface-enhanced Raman spectroscopy (SERS) method coupled with AuNS to detect pesticide residues in juice products. AuNS are multi-branched three-dimensional metal nanostructures with rough surface features that can induce surface plasmon resonance. A facile synthesis of AuNS was achieved using a two-step method and as-prepared AuNS had much cleaner surfaces than gold nanoparticles. A Raman reporter molecule (4-aminothiophenol) was used to evaluate the performance of the SERS method, yielding fingerprint-like Raman spectra and the sensitivity of the SERS method could reach 10 [mu]g/kg. SERS coupled with AuNS was used to detect thiram residues in peach juice and the detection limit was 50 [mu]g/kg, which is 100 times more sensitive than using normal gold nanoparticles. These results demonstrate that AuNS are excellent substrates for SERS measurement, which has great potential for rapid detection of chemical contaminants in food products. This study also aimed to develop a swab method that utilized a nanocellulose-based substrate for SERS applications. The cellulose nanofiber (CNF)-based SERS wiper was developed on a quartz paper by fabricating the quartz/mixture (CNF/silver nanoparticle/AuNS) substrate. A "drop-wipe test" process was used for rapid detection of food contaminants in vegetables by SERS. A Raman probe (4-ATP) was used to evaluate the performance and the sensitivity of SERS/wiper method. The intensity of Raman scattering signals obtained from the quartz/mixture substrate was much higher than the paper/mixture substrate. A stability test was conducted and the results show that in a vacuum package, this new substrate could maintain a high SERS sensitivity for 1 month. The SERS method coupled with the quartz/mixture substrate was used to detect ferbam on kale leaves and the detection limit was 0.05 mg/kg. The measurement of one sample can be finished within a few minutes. These results demonstrate that this novel nanocellulose-based SERS wiper can be used as an excellent substrate for the rapid detection of chemical contaminants in fresh produce.
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