Application of surface enhanced Raman scattering to the solution based detection of a popular legal high, 5,6-methylenedioxy-2-aminoindane (MDAI)

The ever increasing numbers and users of designer drugs means that analytical techniques have to evolve constantly to facilitate their identification and detection. We report that surface enhanced Raman scattering (SERS) offers a relatively fast and inexpensive method for the detection of MDAI at low concentrations. Careful optimisation of the silver sol, and salt concentrations was undertaken to ensure the SERS analysis was both reproducible and sensitive. The optimised system demonstrated acceptable peak variations of less than 15% RSD and resulted in a detection limit of just 8 ppm (5.4 × 10-5 M)

Analysis of photothermal release of Oligonucleotides from hollow Gold nanospheres by surface enhanced raman scattering (SERS)

The photothermal release of single stranded DNA (ssDNA) from the surface of gold nanoparticles of different shapes and sizes is a promising mode of delivering DNA for gene-therapy applications. Here, we demonstrate the first targeted photothermal release of ssDNA from hollow gold nanospheres (HGNs) and analyse the release of the ssDNA using quantitative surface enhanced Raman scattering (SERS). The HGNs used demonstrate a tunable localized surface plasmon resonance (LSPR) frequency while maintaining size consistency, allowing for selective ssDNA release based on matching the excitation frequency to the plasmon resonance. It is shown that HGNs with resonances at 760 and HGN 670 nm release significant amounts of ssDNA when excited via 785 nm and 640 nm lasers respectively. When excited with a wavelength far from the LSPR of the particles, the ssDNA release is negligible. This is the first demonstration of SERS to analyze the amount of ssDNA photothermally released from the surface of HGNs. In contrast to traditional fluorescence measurements, this SERS based approach provides quantitatively robust data for analysis of ssDNA release and lays a strong foundation for future studies exploiting plasmonically induced ssDNA release

SERS active colloidal nanoparticles for the detection of small blood biomarkers using aptamers

Functionalized colloidal nanoparticles for SERS serve as a promising multifunctional assay component for blood biomarker detection. Proper design of these nanoprobes through conjugation to spectral tags, protective polymers, and sensing ligands can provide experimental control over the sensitivity, range, reproducibility, particle stability, and integration with biorecognition assays. Additionally, the optical properties and degree of electromagnetic SERS signal enhancement can be altered and monitored through tuning the nanoparticle shape, size, material and the colloid's local surface plasmon resonance (LSPR). Aptamers, synthetic affinity ligands derived from nucleic acids, provide a number of advantages for biorecognition of small molecules and toxins with low immunogenicity. DNA aptamers are simpler and more economical to produce at large scale, are capable of greater specificity and affinity than antibodies, are easily tailored to specific functional groups, can be used to tune inter-particle distance and shift the LSPR, and their intrinsic negative charge can be utilized for additional particle stability.1,2 Herein, a "turn-off" competitive binding assay platform involving two different plasmonic nanoparticles for the detection of the toxin bisphenol A (BPA) using SERS is presented. A derivative of the toxin is immobilized onto a silver coated magnetic nanoparticle (Ag@MNP), and a second solid silver nanoparticle (AgNP) is functionalized with the BPA aptamer and a Raman reporter molecule (RRM). The capture (Ag@MNP) and probe (AgNP) particles are mixed and the aptamer binding interaction draws the nanoparticles closer together, forming an assembly that results in an increased SERS signal intensity. This aptamer mediated assembly of the two nanoparticles results in a 100x enhancement of the SERS signal intensity from the RRM. These pre-bound aptamer/nanoparticle conjugates were then exposed to BPA in free solution and the competitive binding event was monitored by the decrease in SERS intensity

Objective assessment of SERS thin films : Comparison of silver on copper: via galvanic displacement with commercially available fabricated substrates

Many studies report the development of new thin films for surface enhanced Raman scattering (SERS). However, the assessment of these surfaces in terms of their reproducibility for SERS is often subjective and whilst many spectra could and indeed should be reported, very few repeat measurements are typically used. Here, the performance of three SERS thin film substrates is assessed objectively using both univariate and novel multivariate methods. The silver on copper substrate (SoC) was synthesised in-house via galvanic displacement, whilst the other two substrates Klarite and QSERS are commercially available. The reproducibility of these substrates was assessed using rhodamine 6G (R6G) as a probe analyte and seven common vibrational bands that were observed in all R6G spectra were evaluated. In order to be as objective as possible a total of seven different data analysis methods were used to evaluate the surfaces revealing that overall the SoC substrate demonstrates much greater reproducibility when compared to the commercial substrates. Finally, through the collection of large datasets containing 6400 spectra per single substrate we also provide guidelines as to the typical number of spectra that should be collected in order to assess a substrate's performance objectively, and we conclude that this must be a minimum of 180 spectra collected randomly from across the region of interest

From synthetic DNA to PCR product : detection of fungal infections using SERS

We report the use of silver hydroxylamine nanoparticles functionalised with single stranded monothiolated DNA for the detection of fungal infections. The four different species of fungi that were targeted were Candida albicans, Candida glabrata, Candida krusei and Aspergillus fumigatus. Rational design of synthetic targets and probes was carried out by carefully analysing the 2-D folding of the DNA and then by global alignment of the sequences to ensure specificity. The effects of varying the concentrations of the DNA and dye surrounding the nanoparticles on the resultant surface enhanced Raman scattering (SERS) signal was also investigated to ensure compatibility of the probes in a multiplexed environment. Using principal components analysis (PCA) it was possible to detect the presence of the individual presence of each target and group them accordingly. The move to detect the C. krusei single stranded PCR product (ssPCR) was significant to demonstrate that the methodology could be employed for the detection and diagnosis of invasive fungal infections (IFDs) within a clinical setting. Initially the PCR product was subjected to an alkali shock method in order to separate the strands ready for detection using the nanoparticle probes system. This time 18 base probes were employed to enhance hybridisation efficiency and dextran sulfate was found to have a vital role in ensuring that detection of the C. krusei target was achieved. This demonstrated the use of DNA functionalised silver nanoparticle for detection of clinically relevant DNA relating to a specific fungal infection and offers significant promise for future diagnostic applications

Engineering molecularly-active nanoplasmonic surfaces for DNA detection via colorimetry and Raman scattering

We report a novel nanophotonic biosensor surface capable of both colorimetric detection and Raman-scattered detection of DNA infection markers at extreme sensitivities. Combining direct-write lithography, dip-pen nanolithography based DNA patterning, and molecular self-assembly, we create molecularly-active plasmonic nanostructures onto which metallic nanoparticles are located via DNA-hybridization. Arraying these structures enables optical surfaces that change state when contacted by specific DNA sequences; shifting the surface color while simultaneously generating strong Raman-scattering signals. Patterning the DNA markers onto the plasmonic surface as micro-scale symbols results in easily identifiable color shifts, making this technique applicable to multiplexed lab-on-a-chip and point-of-care diagnostic applications

Determination of metal ion concentrations by SERS using 2,2’-bipyridyl complexes

Surface enhanced Raman scattering (SERS) can generate characteristic spectral “fingerprints” from metal complexes, thus providing the potential for the development of methods of analysis for the identification and quantitation of a range of metal ions in solution. The advantages include sensitivity and the use of one ligand for several metals without the need for a specific chromophore. Aqueous solutions of Fe(II), Ni(II), Zn(II), Cu(II), Cr(III) and Cd(II) in the presence of excess 2,2′-bipyridyl (bipy) were analysed using SERS. Specific marker bands enabled the identification of each metal ion and the limit of detection for each metal ion was estimated. Two of the ions, Zn(II) and Cu(II), could be detected below the World Health Organisation's (WHO) recommended limits for drinking water at levels of 0.22 and 0.6 mg L−1, respectively

Detection of potentially toxic metals by SERS using salen complexes

Surfaced enhanced Raman scattering (SERS) can discriminate between metal complexes due to the characteristic “spectral fingerprints” obtained. As a result, SERS has the potential to develop relatively simple and sensitive methods of detecting and quantifying a range of metal ions in solution. This could be beneficial for the environmental monitoring of potentially toxic metals (PTMs). Here, salen (C16H16N2O2) was used as a ligand to form complexes of Ni(II), Cu(II), Mn(II) and Co(II) in solution. The SERS spectra showed characteristic spectral differences specific to each metal complex, thus allowing the identification of each of these metal ions. This method allows a number of metal ions to be detected using the same ligand and an identical preparation procedure. The limit of detection (LOD) was determined for each metal ion, and it was found that Ni(II), Cu(II) and Mn(II) could be detected below the WHO’s recommended limits in drinking water at 1, 2 and 2 µg L-1, respectively. Co(II) was found to have an LOD of 20 µg L-1, however no limit has been set for this ion by the WHO as the concentration of Co(II) in drinking water is generally <1-2 μg L-1. A contaminated water sample was also analysed where Mn(II) was detected at a level of 800 µg L-1

Comparison of Fe2O3 and Fe2CoO4 core-shell plasmonic nanoparticles for aptamer mediated SERS assays

Conjugation of oligonucleotides or aptamers and their corresponding analytes onto plasmonic nanoparticles mediates the formation of nanoparticle assemblies: molecularly bound bundles of nanoparticles which cause a measurable change in the colloid's optical properties. Here, we present further optimization of a "SERS off" competitive binding assay utilizing plasmonic and magnetic nanoparticles for the detection of the toxin bisphenol A (BPA). The assay involves 1) a 'target' silver nanoparticle functionalized with a Raman reporter dye and PEGylated BPA-binding DNA aptamers, and 2) a version of the toxin BPA, bisphenol A diglycidyl ether (BADGE), PEGylated and immobilized onto a silver coated magnetic 'probe' nanoparticle. When mixed, these target and probe nanoparticles cluster into magnetic dimers and trimers and an enhancement in their SERS spectra is observed. Upon introduction of free BPA in its native form, target AgNPs are competitively freed; reversing the nanoparticle assembly and causing the SERS signal to "turn-off" and decrease in response to the competitive binding event. The assay particles were housed inside two types of optofluidic chips containing magnetically active nickel pads, in either a straight or spotted pattern, and both Fe2O3 and Fe2CoO4 were compared as magnetic cores for the silver coated probe nanoparticle. We found that the Ag@ Fe2O3 particles were, on average, more uniform in size and more stable than Ag@ Fe2CoO4, while the addition of cobalt significantly improved the collection time of particles within the magnetic chips. Using 3D Raman mapping, we found that the straight channel design with the Ag@ Fe2O3 particles provided the most uniform nanoparticle organization, while the spotted channel design with Ag@ Fe2CoO4 demonstrated a larger SERS enhancement, and thus a lower limit of detection

Through tissue imaging of a live breast cancer tumour model using handheld surface enhanced spatially offset Resonance Raman Spectroscopy

In order to improve patient survival and reduce the amount of unnecessary and traumatic biopsies, non-invasive detection of cancerous tumours is of imperative and urgent need. Multicellular tumour spheroids (MTS) can be used as an ex vivo cancer tumour model, to model in vivo nanoparticle (NP) uptake by the enhanced permeability and retention (EPR) effect. Surface enhanced spatially offset Raman spectroscopy (SESORS) combines both surface enhanced Raman spectroscopy (SERS) and spatially offset Raman spectroscopy (SORS) to yield enhanced Raman signals at much greater sub-surface levels. By utilizing a reporter that has an electronic transition in resonance with the laser frequency, surface enhanced resonance Raman scattering (SERRS) yields even greater enhancement in Raman signal. Using a handheld SORS spectrometer with back scattering optics, we demonstrate the detection of live breast cancer 3D multicellular tumour spheroids (MTS) containing SERRS active NPs through 15 mm of porcine tissue. False color 2D heat intensity maps were used to determine tumour model location. In addition, we demonstrate the tracking of SERRS-active NPs through porcine tissue to depths of up to 25 mm. This unprecedented performance is due to the use of red-shifted chalcogenpyrylium-based Raman reporters to demonstrate the novel technique of surface enhanced spatially offset resonance Raman spectroscopy (SESORRS) for the first time. Our results demonstrate a significant step forward in the ability to detect vibrational fingerprints from a tumour model at depth through tissue. Such an approach offers significant promise for the translation of NPs into clinical applications for non-invasive disease diagnostics based on this new chemical principle of measurement