Detection of Listeria innocua on roll-to-roll produced SERS substrates with gold nanoparticles

Abstract

The rapid and accurate detection of food pathogens plays a critical role in the early prevention of foodborne epidemics.Current bacteria identification practises, including colony counting, polymerase chain reaction (PCR) and immunological methods, are time consuming and labour intensive; they are not ideal for achieving the required immediate diagnosis. Different SERS substrates have been studied for the detection of foodborne microbes. The majority of the approaches are either based on costly patterning techniques on silicon or glass wafers or on methods which have not been tested in large scale fabrication. We demonstrate the feasibility of analyte specific sensing using mass-produced, polymer-based low-cost SERS substrate in analysing the chosen model microbe with biological recognition. The use of this novel roll-to-roll fabricated SERS substrate was combined with optimised gold nanoparticles to increase the detection sensitivity.Distinctive SERS spectral bands were recorded for Listeria innocua ATCC 33090 using an in-house build (785 nm) near infra red (NIR) Raman system. Results were compared to both those found in the literature and the results obtained from a commercial time-gated Raman system with a 532 nm wavelength laser excitation. The effect of the SERS enhancer metal and the excitation wavelength on the detected spectra was found to be negligible. The hypothesis that disagreements within the literature regarding bacterial spectral results from conditions present during the detection process has not been supported. The sensitivity of our SERS detection was improved through optimization of the concentration of the sample inside the hydrophobic polydimethylsiloxane (PDMS) wells. Immuno-magnetic separation (IMS) beads were used to assist the accumulation of bacteria into the path of the beam of the excitation laser. With this combination we have detected L i s t e r i a w i t h g o l d e n h a n c e d S E R S i n a l a b e l f r e e m a n n e r f r o m s u c h l o w s a m p l e c o n c e n t r a t i o n s a s 1 0 4 CFU/ml.Figure 10. a) A normalised concentration series for LOD estimation. b) An exponential fit for the normalised concentration series in logarithmic scale for the entire series and a linear fit for the small concentrations. c) Comparison of the 737 cm -1 peak intensity for different concentration series with 5 -10 µl dried IMS bound L. innocua ATCC 33090 samples placed with AuNPs into a 1 -1.5 mm PDMS well on top of SERS substrate. d) Comparison of baseline corrected Raman intensities for three of the concentration series. All figures are a mean of 9 measurement points with mean absolute deviations.

Bare laser‐synthesized Au‐based nanoparticles as nondisturbing surface‐enhanced Raman scattering probes for bacteria identification

“…Furthermore, using methods of ultrashort laser ablation or fragmentation , one can prepare extremely stable aqueous solutions of “bare” (ligand‐free) Au NPs having unique surface chemistry and different reactivity compared to conventional colloidal nanomaterials. The absence of ligands and contaminants on the surface of such Au‐based NPs gives a promise for their successful use as mobile probes for SERS applications and their efficiency in such tasks was recently confirmed in our studies . Moreover, despite the absence of any biopolymer coating such “bare” Au NPs were found to have relatively low toxicity and a good cell uptake, which promises their successful use in live systems .…”

“…The beads were concentrated, the supernatant was removed and the captured bacteria with beads were resuspended into 100 mL of washing buffer for the SERS detection. The blank Raman spectrum of immunomagnetic separation (IMS) beads in LEE Broth can be found in our earlier study . Bacteria spectra were detected with 785 nm laser excitation, 60 seconds integration time, 50× objective and 7.5 mW power.…”

Comparison of time‐gated surface‐enhanced raman spectroscopy (TG‐SERS) and classical SERS based monitoring of Escherichia coli cultivation samples

“…Prior to measurements, the stock solution was centrifuged (CT15RE from VWR, Radnor, PA) at 4,000 rpm/1,610 g for 4 min, subsequently followed by a removal of the supernatant to reach a final concentration of about 2.8 g·L −1 . The effect of different size, shape and enhancement material was tested in our earlier studies, in which these Ag NPs showed good performance at different wavelengths with different spectroscopic set‐ups . The nanotoxicity of Ag NPs on E. coli cells was described elsewhere, which should not be relevant, as the assay was exclusively used during measurements.…”

“…The effect of different size, shape and enhancement material was tested in our earlier studies, in which these Ag NPs showed good performance at different wavelengths with different spectroscopic set-ups. 24,25 The nanotoxicity of Ag NPs on E. coli cells was described elsewhere, 26 which should not be relevant, as the assay was exclusively used during measurements.…”

Rapid detection of Listeria monocytogenes using fluorescence immunochromatographic assay combined with immunomagnetic separation technique

“…Molecular biology methods, such as nucleic acid probe and gene chip, have high sensitivity, but need expensive equipment and skilled operation (Feng et al ., ; Skerniškyt≐ et al ., ). Various types of immunological detection methods like enzyme‐linked immunosorbent assay [(ELISA) Mendonça et al ., ; Mizutani et al ., ;], colloidal gold immunochromatography (Shan et al ., ) and immunomagnetic separation (IMS) method (Uusitalo et al ., ) have widely been used in pathogens and toxins detection. Among all immunological methods, immunochromatography is the simplest and quickest way to detect the pathogen in a sample within 15 min (Shim et al ., ).…”