Aluminum Foil vs. Gold Film: Cost-Effective Substrate in Sandwich SERS Immunoassays of Biomarkers Reveals Potential for Selectivity Improvement.

The first application of aluminum foil (Al F) as a low-cost/high-availability substrate for sandwich immunoassay using surface-enhanced Raman spectroscopy (SERS) is reported. Untreated and unmodified Al F and gold film are used as substrates for sandwich SERS immunoassay to detect tuberculosis biomarker MPT64 and human immunoglobulin (hIgG) in less than 24 h. The limits of detection (LODs) for tuberculosis (TB) biomarker MPT64 on Al foil, obtained with commercial antibodies, are about 1.8-1.9 ng/mL, which is comparable to the best LOD (2.1 ng/mL) reported in the literature for sandwich ELISA, made with fresh in-house antibodies. Not only is Al foil competitive with traditional SERS substrate gold for the sandwich SERS immunoassay in terms of LOD, which is in the range 18-30 pM or less than 1 pmol of human IgG, but it also has a large cost/availability advantage over gold film. Moreover, human IgG assays on Al foil and Si showed better selectivity (by about 30-70% on Al foil and at least eightfold on Si) and a nonspecific response to rat or rabbit IgG, in comparison to the selectivity in assays using gold film.

High Contrast Surface Enhanced Fluorescence of Carbon Dot Labeled Bacteria Cells on Aluminum Foil.

Surface enhanced fluorescence (SEF) is observed with very high contrast (100-200) from single E. coli bacteria cells labeled with Carbon nanodots (CDs), on aluminum foil and aluminum film. Likely, it is the first application of organic CDs in SEF. SEF with 633 nm excitation delivered a much higher contrast than SEF with 532 nm excitation. Contrast is the ratio of the fluorescent intensities of labeled CDs to unlabeled (control) cells. High contrast with CDs is also observed on the gold film, silicon, and glass. Enhancement factor (EF) is the ratio of the signal on the metal substrate to the signal on the glass. Single E. coli cells, labeled with commercial graphene quantum dots (GCDs), demonstrated higher EFs (44 on gold, 35 on Al film), but at least one order of magnitude lower contrast (7-10 on aluminum and gold) than cells labeled with organic CDs. Therefore, organic CDs can be a good choice for cell imaging/labeling, capable of achieving a signal to noise (standard deviation of the control) as high as 700 on Al film. Overall, aluminum foil and film are highlighted as inexpensive but efficient substrates for Metal Enhanced Fluorescence, particularly MEF of bacterial cells stained with CDs.

How gap distance between gold nanoparticles in dimers and trimers on metallic and non-metallic SERS substrates can impact signal enhancement.

The impact of variation in the interparticle gaps in dimers and trimers of gold nanoparticles (AuNPs), modified with Raman reporter (2-MOTP), on surface-enhanced Raman scattering (SERS) intensity, relative to the SERS intensity of a single AuNP, is investigated in this paper. The dimers, trimers, and single particles are investigated on the surfaces of four substrates: gold (Au), aluminium (Al), silver (Ag) film, and silicon (Si) wafer. The interparticle distance between AuNPs was tuned by selecting mercaptocarboxylic acids of various carbon chain lengths when each acid forms a mixed SAM with 2-MOTP. The SERS signal quantification was accomplished by combining maps of SERS intensity from a Raman microscope, optical microscope images (×100), and maps/images from AFM or SEM. In total, we analysed 1224 SERS nanoantennas (533 dimers, 648 monomers, and 43 trimers). The average interparticle gaps were measured using TEM. We observed inverse exponential trends for the Raman intensity ratio and enhancement factor ratio versus gap distance on all substrates. Gold substrate, followed by silicon, showed the highest Raman intensity ratio (9) and dimer vs. monomer enhancement factor ratio (up to 4.5), in addition to the steepest inverse exponential curve. The results may help find a balance between SERS signal reproducibility and signal intensity that would be beneficial for future agglomerated NPs in SERS measurements. The developed method of 3 to 1 map combination by an increase in image transparency can be used to study structure-activity relationships on various substrates in situ, and it can be applied beyond SERS microscopy.