The exploration of nanomaterials with mimic enzyme activity (named nanozyme) has gained extensive attention in the fields of advanced analytical chemistry and materials science. Herein, the gold nanoparticles doped covalent organic frameworks (COFs) were prepared, which exhibited not only excellent mimic nitroreductase activity but also robust stability. By replacing the traditional natural enzyme tag in an enzyme-linked immunosorbent assay (ELISA), we employed the proposed nanozyme to label the detecting antibody. According to the catalytic properties of the nanozyme, 4-nitrothiophenol (4-NTP) was introduced as the substrate, which can be transformed to 4-aminothiophenol (4-ATP) in the presence of NaBH 4 . In a surface enhanced Raman scattering (SERS) assay, 4-ATP was capable of functioning as a powerful bridge to connect the gold nanostars (with excellent SERS performance) by both the Au−S bond and electrostatic force to further produce a Raman "hot spot". Meanwhile, the Raman signal of 4-nitrothiophenol at 1573 cm −1 was weakened, and a new signal at 1591 cm −1 generated by 4-ATP was turned on, leading to the generation of a ratiometric SERS signal. Based on this performance, a ratiometric nanozymelinked immunosorbent assay (NELISA) strategy was developed delicately, which was applied to detect β-lactoglobulin (allergenic protein) by monitoring the ratiometric signal of I 1591 /I 1573 with a limit of detection (LOD) of 0.01 ng/mL. The linear range is 25.65−6.2 × 10 4 ng/mL, covering more than 3 orders of magnitude. The developed method showed many advantages such as low-cost, higher recovery, and lower cross-reactivity, providing new insight into the application of SERS technology for trace target analysis.
A pragmatic method to deposit silver nanoparticles on polydopamine-coated nanoimprinted pillars for use as surface-enhanced Raman scattering (SERS) substrates was developed. Pillar arrays consisting of poly(methyl methacrylate) (PMMA) that ranged in diameter from 300 to 500 nm were fabricated using nanoimprint lithography. The arrays had periodicities from 0.6 to 4.0 μm. A polydopamine layer was coated on the pillars in order to facilitate the reduction of silver ions to create silver nucleation sites during the electroless deposition of sliver nanoparticles. The size and density of silver nanoparticles were controlled by adjusting the growth time for the optimization of the SERS performance. The size of the surface-adhered nanoparticles ranged between 75 and 175 nm, and the average particle density was ∼30 particles per μm(2). These functionalized arrays had a high sensitivity and excellent signal reproducibility for the SERS-based detection of 4-methoxybenzoic acid. The substrates were also able to allow the SERS-based differentiation of three types of bacteriophages (λ, T3, and T7).
Here we presented a simple, rapid and label-free surface-enhanced Raman spectroscopy (SERS) based mapping method for the detection and discrimination of Salmonella enterica and Escherichia coli on silver dendrites. The sample preparation was first optimized to maximize sensitivity. The mapping method was then used to scan through the bacterial cells adsorbed on the surface of silver dendrites. The intrinsic and distinct SERS signals of bacterial cells were used as the basis for label-free detection and discrimination. The results show the developed method is able to detect single bacterial cells adsorbed on the silver dendrites with a limit of detection as low as 10(4) CFU mL(-1), which is two orders of magnitude lower than the traditional SERS method under the same experimental condition. The time needed for collecting a 225 points map was approximately 24 minutes. Moreover, the developed SERS mapping method can realize simultaneous detection and identification of Salmonella enterica subsp. enterica BAA1045 and Escherichia coli BL21 from a mixture sample using principle component analysis. Our results demonstrate the great potential of the label-free SERS mapping method to detect, identify and quantify bacteria and bacterial mixtures simultaneously.
An ultrasensitive surface-enhanced Raman scattering (SERS) biosensor driven by CRISPR/Cas12a was proposed for on-site nucleic acid detection. We tactfully modified single-strand DNA (ssDNA) with a target-responsive Prussian blue (PB) nanolabel to form a probe and fastened it in the microplate. Attributed to the specific base pairing and highly efficient trans-cleavage ability of the CRISPR/Cas12a effector, precise target DNA recognition and signal amplification can be achieved, respectively. In the presence of target DNA, trans-cleavage towards the probe was activated, leading to the release of a certain number of PB nanoparticles (NPs). Then, these free PB NPs would be removed. Under alkali treatment, the breakdown of the remaining PB NPs in the microplate was triggered, producing massive ferricyanide anions (Fe(CN) 6 4− ), which could exhibit a unique characteristic Raman peak that was located in the "biological Raman-silent region". By mixing the alkali-treated solution with the SERS substrate, Au@Ag core−shell NP, the concentration of the target DNA was finally exhibited as SERS signals with undisturbed background, which can be detected by a portable Raman spectrometer. Importantly, this strategy could display an ultralow detection limit of 224 aM for target DNA. Furthermore, by targeting cow milk as the adulterated ingredient in goat milk, the proposed biosensor was successfully applied to milk authenticity detection.
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