Determination of biomarkers in clinical or food samples is of crucial importance for monitoring, prevention, and care of public health. The standard procedure used for this purpose is the enzyme-linked immunosorbent assay (ELISA), which makes use of the specific antibody–antigen biorecognition and the catalytic effect of the enzymes. One of the main shortcomings of this technique is the use of enzymes that often present low chemical and thermal stabilities compared to other chemicals. Other drawbacks include the nonspecific binding process that could lead to false-positive results, the use of relatively large amounts of the sample, and the number of time-consuming steps involved. Recently, an enzyme-free and ultrasensitive analytical method for antigen detection denoted as intensity depletion immunolinked assay (IDILA) has been proposed by our laboratory. The assay is based on the inhibition to form Ag nanosphere dimers linked by a specific antibody in the presence of the corresponding antigen. In this work, we go a step further demonstrating how the performance of this method could be improved by using silver nanoparticles (Ag NPs) of different diameters (58 and 78 nm). The experiments are performed for detecting gliadin, an antigen of utmost importance in celiac disease, and the results are compared with ELISA, the standard technique homologated by the Food Codex Alimentarius. It is found that the IDILA assay could be around 1000 or 10 000 times more sensitive than ELISA, also having lower limits of detection, depending on the conditions explored (fraction of dimers and Ag NP diameter). Using the appropriate conditions, the IDILA assay is shown to be able to detect femtomolar concentrations of the antigen, besides being robust, reliable, cheap, rapid (around 2 h), and of easy implementation using the standard equipment and biomolecular reagents used for the ELISA assay.

Plasmonic nanoparticle aggregates are one of the most widely employed nanostructures as colloidal surface enhanced Raman spectroscopy (SERS) substrates mainly due to their ability to generate huge near-field enhancements. However, the available definitions to determine the enhancement factor, a key parameter to quantitatively describe the quality of SERS substrates, exhibit important limitations to adequately describe the performance of colloidal plasmonic nanoparticle aggregates as SERS platforms. Herein, we introduce a new figure of merit named active concentration enhancement factor (ACEF) to assess the SERS enhancement factor of colloidal gold nanoparticle aggregates, which shows significant improvements to characterize the SERS performance with respect to formerly reported parameters. The determination of the analyte active concentration, that is, the concentration of analyte molecules that effectively contributes to the SERS signal, was achieved according to a strategy that involves a rigorous modeling of the extinction spectra of the colloidal gold nanoparticle aggregates. Furthermore, the experimentally obtained ACEF values can be directly compared with theoretically calculated electromagnetic field enhancement factors. We believe that the figure of merit introduced might help to strengthen the development and design of SERS substrates.

Surface-enhanced Raman spectroscopy (SERS) has demonstrated to be a powerful technique for the ultrasensitive detection of different types of analytes and particularly biomolecules, with the rational design of SERS substrates being one of the most relevant issues for the development of effective detection protocols. In this work, a colloidal SERS substrate consisting of a pair of Au nanorods linked by the molecular bridge biotin/streptavidin/biotin has been obtained and employed for the detection of picomolar quantities of the biotinylated antibodies gliadin IgG and Ara h1 IgG, which is of great interest in food science. As a consequence of the bioconjugation strategy implemented, the SERS substrate, that is, the Au nanorod dimer, presents the advantage of combining in a single nanostructure the capabilities for both direct and indirect detection of the biotinylated antibodies. Furthermore, the experimental results are supported by detailed electrodynamics simulations which takes into account not only the gradient of the near-field enhancements within the hot spot but also the volume occupied by the respective biomolecules. The SERS substrate presented here could be straightforwardly employed for the ultrasensitive detection of other biotinylated biomolecules.

Gold nanorods (GNRs) coated with mesoporous silica (GNRs@m-SiO 2 ) have proven to be a robust nanostructure with useful applications in biomedical, catalysis, and molecular sensing areas, among others. The m-SiO 2 shell improves the nanoparticle stability and grants a concomitant molecular loading capability. One of the factors that determine the specific application of GNRs@m-SiO 2 is the porosity degree of the m-SiO 2 shell. In the present work, we first studied how the extinction spectra features of GNRs@m-SiO 2 in combination with electrodynamics modeling can be used to determine the porosity degree of the m-SiO 2 shell produced at two different concentrations of the porogenic surfactant cethyl trimethylammonium bromide (CTAB). The changes on the intensity in the lowfrequency region are explained qualitatively in terms of the optical properties of the mesoporous silica spheres formed as byproducts. Varying the CTAB concentration produces a change not only on the porosity but also on the thickness of the m-SiO 2 shell. With rigorous discrete dipole approximation (DDA) simulations, together with an effective medium approach (Maxwell-Garnet) for the m-SiO 2 , it is demonstrated that the peak position of the longitudinal localized surface plasmon resonance (LSPR) plasmon mode depends only on the effective dielectric constant of the m-SiO 2 shell (assuming that all the pores are filled by water). The volume fraction of water in the m-SiO 2 shell that, in the DDA simulations, fits the peak position of the longitudinal LSPR of the experimental extinction spectra is a measure of the m-SiO 2 shell porosity. DDA simulations show that GNRs@m-SiO 2 fabricated with the highest CTAB concentration depicts a degree porosity high enough to allow the diffusion of an analyte toward the GNR core. This feature was tested by determining the analytical surface-enhanced Raman spectroscopy (SERS) enhancement factor of Rhodamine 6G as a molecular probe and comparing it with a theoretical SERS enhancement factor in different regions around the GNRs@m-SiO 2 structure. Second, we introduce a simple approach, denoted as quasi-static effective medium approach (QSEMA) for determining the resonance condition of the longitudinal LSPR for core−shell prolate spheroids and therefore the shell porosity. This simple approach leads to the same results as those of rigorous DDA simulations using the exact model geometry. Finally, using QSEMA, we determined the uptake through the pores of each component in glycerin-water solvent mixtures of different compositions.

No abstract

Ionic gels from eutectic mixtures are attracting extensive interest in bioelectronics owing to their nonvolatile nature, low cost, and inherently high ionic conductivity. Biodegradable electronics made of biopolymers envisage a promising future in this field, but unfortunately, they often feature poor mechanics. Herein, we explored tannic acid-decorated cellulose nanocrystals (TA@CNC) as dynamic nanofillers of biocompatible eutectogels based on gelatin and a eutectic mixture composed of choline chloride and ethylene glycol (ethaline). Small concentrations of TA@CNC (up to 1–2 wt %) allow increasing by two-fold the strength (30 kPa) and stretchability (180%) of the eutectogels while improving their ionic conductivity (105 mS·m–1). The reversible physical network of the protein and multiple hydrogen bonding interactions with tannic acid endow these eutectogels with good self-adhesiveness, suitable gel-to-sol transition for 3D printing, and recyclability. We further used the cellulose nanocomposite eutectogels as skin-conformal electrodes for monitoring different motions of the human body with excellent sensitivity in the open air thanks to the low volatility of ethaline. All in all, these results demonstrate a facile strategy to boost the properties of biopolymer eutectogels using inexpensive and renewable raw materials as rigid nanoreinforcers.

Hybrid nanostructures composed of magnetic iron oxides and plasmonic metals are able to convert light energy into chemical energy as well as they can be easily manipulated through magnetic fields. As a consequence of these multifunctional features they can be employed as magnetically recyclable heterogeneous photocatalysts. Herein, we report a two step method for the preparation of magnetite (Fe 3 O 4)-gold (Au) hybrid nanostructures in aqueous media. The obtained material resembles a core-satellite morphology of 60 nm Fe 3 O 4 nanoparticles surrounded by nearly spherical 20 nm Au nanoparticles attached to their surface. The synthesized hybrid material exhibits enhanced capabilities for the methylene blue photodegradation compared with bare Fe 3 O 4 nanoparticles. Detailed electrodynamics simulations were performed to achieve further insight into the improved photoactive properties of the Fe 3 O 4-Au hybrid nanostructures. The theoretical results show that the excitation of localized surface plasmon resonances in the Au component leads to greater light absorption in the Fe 3 O 4 component which ultimately impacts on the improved photocatalytic properties of the hybrid nanostructure. Overall, this work provides a complementary approach toward a complete understanding of the enhanced photoactive properties of hybrid nanostructures and highlights the importance of considering their actual morphology into simulations.

In recent work, a narrow distribution of nanoparticle (NP) aggregates in terms of the number of NPs and controlled interparticle distances has been obtained inducing NP aggregation with very low concentrations of a molecular linker, demonstrating that it is possible to determine the concentrations of the different aggregates using a suitable simulation of the extinction spectra. Herein, we introduce a new quantity denoted the single hot spot active concentration enhancement factor (SHSACEF) to assess in a rigorous way the surface-enhanced Raman spectroscopy (SERS) enhancement contribution. The SHSACEF discriminates the relative contributions to the overall SERS signal from the molecules located in each hot spot (HS) in a colloidal sample consisting of a mixture of NP aggregates of different sizes. The capabilities of this method is tested by inducing Au NP aggregation using a supramolecular complex (cucurbit[6]uril and methylviologen) and comparing the experimental values of the SHSACEF with theoretical simulations of the maximum enhancement generated for an individual HS in each nanoaggregate.