Controlled Permeability in Porous Polymer Nanocapsules Enabling Size- and Charge-Selective SERS Nanoprobes

Micromolding-Assisted Production of SERS-Active Microcylinders for Size- and Charge-Selective Molecular Detection

Surface-enhanced Raman scattering (SERS) is an effective technique for amplifying the Raman signal of molecules by using metal nanostructures. However, these metal surfaces are susceptible to contamination by undesirable adhesives in complex mixtures, typically necessitating a time-consuming and costly sample pretreatment. In order to circumvent this, metal nanoparticles have been uniformly embedded within microgels by using microfluidics. In this work, we introduce a simple, scalable micromolding method for creating SERS-active cylindrical microgels designed to eliminate the need for pretreatment. These microcylinders are created through the simultaneous photoreduction and photo-cross-linking of precursor solutions. These solutions are optimized for consistent, high-intensity Raman signals as well as molecular size and charge selectivity. A sequential micromolding method is employed to design dual-compartment microcylinders, offering additional functionalities such as optical encoding, magnetoresponsiveness, and dual-charge selectivity. These SERS-active microcylinders provide robust Raman signals of small molecules, even in the presence of adhesive proteins, without compromising sensitivity. To demonstrate this capability, we directly detect pyocyanin in saliva and tartrazine in whole milk without any need for sample pretreatment.

Stimuli-responsive SERS biosensor for ultrasensitive tetracycline sensing using EDTA-driven PEI@CaCO3 microcapsule and CS@FeMMs

In this work, a stimuli-responsive SERS biosensor was fabricated for tetracycline (TTC) by "signal-on" strategy using (EDTA)-driven polyethyleneimine grafted calcium carbonate (PEI@CaCO₃) microcapsule and chitosan-Fe magnetic microbeads (CS@FeMMs). Initially, aptamer conjugated magnetic-bead CS@FeMMs@Apt with superparamagnetism and excellent biocompatibility was employed as capture probe, which facilitated the rapid and easy magnetic separation. Subsequently, the PEI cross-linked layer and aptamer network layer were constructed onto the outer layer of CaCO₃@4-ATP microcapsule to form sensing probes (PEI@CaCO₃@4-ATP@Apt) via the layer-by-layer assembly method. In the presence of TTC, a sandwich SERS-assay was exploited by aptamer recognition induced target-bridged strategy. When the solution of EDTA was added, the core layer of CaCO₃ would be dissolved quickly, destroying the microcapsule to release 4-ATP. The released 4-ATP could be quantitatively monitored by dripping the supernatant onto the AuNTs@PDMS SERS platform, resulting in a strong Raman "signal-on". Under the optimal conditions, a good linear relationship was established with a correlation coefficient (R²) of 0.9938 and a LOD of 0.03 ng/mL. Additionally, the application capacity of the biosensor to detect TTC was also affirmed in food matrixes, and the results were consistent with the standard ELISA method (P > 0.05). Hence, this SERS biosensor affords extensive application prospects for TTC detection with multiple merits such as high sensitivity, environment friendliness, and high stability.

Plasmonic Au@Ag@mSiO2 Nanorattles for In Situ Imaging of Bacterial Metabolism by Surface-Enhanced Raman Scattering Spectroscopy

It is well known that microbial populations and their interactions are largely influenced by their secreted metabolites. Noninvasive and spatiotemporal monitoring and imaging of such extracellular metabolic byproducts can be correlated with biological phenotypes of interest and provide new insights into the structure and development of microbial communities. Herein, we report a surface-enhanced Raman scattering (SERS) hybrid substrate consisting of plasmonic Au@Ag@mSiO₂ nanorattles for optophysiological monitoring of extracellular metabolism in microbial populations. A key element of the SERS substrate is the mesoporous silica shell encapsulating single plasmonic nanoparticles, which furnishes colloidal stability and molecular sieving capabilities to the engineered nanostructures, thereby realizing robust, sensitive, and reliable measurements. The reported SERS-based approach may be used as a powerful tool for deciphering the role of extracellular metabolites and physicochemical factors in microbial community dynamics and interactions.

Oil-Free Gold Nanobipyramid@Ag Microgels as a Functional SERS Substrate for Direct Detection of Small Molecules in a Complex Sample Matrix

Surface-enhanced Raman scattering (SERS) is a super-sensitive analysis technology based on the target molecular fingerprint information. The enhancement of local electromagnetic field of the SERS substrate would increase the target molecules' Raman intensity which adsorb on the surface of nanoparticles. However, the existing adhesive macromolecules in the complex mixed sample would interfere with the adsorption of small target molecules, and it weakens the Raman intensity of target molecules. Microgels are one of the potential materials to suppress the interference of adhesive macromolecules and to avoid the complex pretreatments. However, most of the current microgel synthesis methods involve complex operations with precise instrumentation or the interference of oil and organic reagents. In this work, a simple and oil-free method was proposed to synthesize the gold nanobipyramid (Au NBP)@Ag@hyaluronic acid microgel via the condensation reaction of carboxyl and amino groups. As a proof-of-concept demonstration for small-molecule detection, the rhodamine 6G (R6G) molecules were allowed to enter inside the microgel through the meshes and adsorb on the surface of Au NBP@Ag nanoparticles within 30 min, while the macromolecule (bovine serum albumin in this case) was retained outside the microgel in the meantime. In addition, under the combined action of lightning rod effect of Au NBP and surface plasmon resonance effect of silver render the microgels with high SERS activity. The synthetic Au NBP@Ag@hyaluronic acid microgels were applied to detect 6-thioguanine in the human serum without any pretreatment process, and it showed a high signal enhancement and stable SERS signal, which can satisfy the requirement of clinical diagnosis. These results show that the proposed microgels have potential applications in the field of point-of-care testing.

Electrokinetic Preseparation and Molecularly Imprinted Trapping for Highly Selective SERS Detection of Charged Phthalate Plasticizers

Nonspecific binding and weak spectral discernment are the main challenges for surface-enhanced Raman scattering (SERS) detection, especially in real sample analysis. Herein, molecularly imprinted polymer (MIP)-based core-shell AuNP@polydopamine (AuNP@PDA-MIP) nanoparticles (NPs) are designed and immobilized on an electrochemically reduced MoS₂-modified screen-printed electrode (SPE). This portable electrochemical-Raman interface offers the dual functions of electrokinetic preseparation (EP) and MIP trapping of charged molecules so that a reliable SERS recognition with molecular selectivity and high sensitivity can be achieved. Core-shell AuNP@PDA-MIP NPs can be controllably synthesized, possess predesigned specific recognition, and provide "hot spots" at the junction of NPs. The introduction of an electric field enables the autonomous exclusion and separation of similarly charged molecules as well as attraction and concentration of the oppositely charged molecules by electrostatic attraction. Subsequently, the specific MIP recognition cavities allow selective adsorption of targets on the interface without the interference of analogues. Owing to the distinctive design of the multiple coupling separation, trapping, and enrichment strategies, the MIP-based SERS-active interface can be used for label-free detection of charged molecules in real samples without pretreatment. As a proof-of-concept study, label-free SERS detection of charged phthalate plasticizers (PAEs) was demonstrated with a detection limit as low as 2.7 × 10^-12 M for dimethyl phthalate (DMP) and 2.3 × 10^-11 M for di(2-ethylhexyl) phthalate (DEHP). This sensing strategy for in situ SERS analysis of charged pollutants or toxins holds vast promises for a wide range of in-field applications.

Plasmonic Microgels for Raman-Based Molecular Detection Created by Simultaneous Photoreduction and Photocross-linking

Molecular detection in complex mixtures is of great importance in biomedical diagnosis, food safety, and environmental monitoring. Although surface-enhanced Raman scattering serves as one of the most promising detection methods, metal surfaces are prone to contamination, making the direct detection of small molecules in mixtures elusive. Metal nanoparticle-loaded hydrogels have been used for the exclusion of large adhesive molecules and direct detection of small molecules. Here, we design microgels containing highly concentrated gold nanoparticles through the simultaneous formation of hydrogel and gold nanoparticles in emulsion droplets. Monodisperse water-in-oil droplets are microfluidically prepared to contain a gold precursor, hydrogel precursor, and photoinitiator. Upon ultraviolet irradiation, a hydrogel is gradually formed in the drop by photocross-linking at which gold nanoparticles are synthesized and grown by photo and thermal reduction. The in situ synthesis provides the uniform distribution of gold nanoparticles at very high concentrations without aggregation, which is otherwise very difficult to achieve. Using the microgels, small molecules in albumin solutions can be detected by Raman measurement with high signal sensitivity and reproducibility in the absence of interruption from albumin. As a proof of concept, we demonstrate the direct detection of pyocyanin, a biomarker for Pseudomonas infection spiked in unpurified saliva.

Bilayer-Templated Two-Dimensional RAFT Polymerization for Directed Assembly of Polymer Nanostructures

Co-localization of monomers, crosslinkers, and chain-transfer agents (CTA) within self-assembled bilayers in an aqueous suspension enabled the successful directed assembly of nanocapsules using a reversible addition-fragmentation chain transfer (RAFT) process without compromising the polymerization kinetics. This study uncovered substantial influence of the organized medium on the course of the reaction, including differential reactivity based on placement and mobility of monomers, crosslinkers, and CTAs within the bilayer.

Mechanism of a Self-Assembling Smart and Electrically Responsive PVDF-Graphene Membrane for Controlled Gas Separation

The development of science and technology is accompanied by a complex composition of multiple pollutants. Conventional passive separation processes are not sufficient for current industrial applications. The advent of active or responsive separation methods has become highly essential for future applications. In this work, we demonstrate the preparation of a smart electrically responsive membrane, a poly(vinylidene difluoride) (PVDF)-graphene composite membrane. The high graphene content induces the self-assembly of PVDF with a high β-phase content, which displays a unique self-piezoelectric property. Additionally, the membrane exhibits excellent electrical conductivity and unique capacitive properties, and the resultant nanochannels in the membrane can be reversibly adjusted by external voltage applications, resulting in the tailored gas selectivity of a single membrane. After the application of voltage to the membrane, the permeability and selectivity toward carbon dioxide increase simultaneously. Moreover, atomic-level positron annihilation spectroscopic studies reveal the piezoelectric effect on the free volume of the membrane, which helps us to formulate a gas permeation mechanism for the electrically responsive membrane. Overall, the novel active membrane separation process proposed in this work opens new avenues for the development of a new generation of responsive membranes.

Encapsulation of 3D plasmonic nanostructures with ultrathin hydrogel skin for rapid and direct detection of toxic small molecules in complex fluids

Nanogap-rich 3D plasmonic nanostructures provide enhanced molecular Raman fingerprints in a nondestructive and label-free manner. However, the molecular detection of small target molecules in complex fluids is challenging due to nonspecific protein adsorption, which prevents access of the target molecules. Therefore, the molecular detection for complex mixtures usually requires a tedious and time-consuming pretreatment of samples. Herein, we report the encapsulation of 3D plasmonic nanostructures with an ultrathin hydrogel skin for the rapid and direct detection of small molecules in complex mixtures. To demonstrate the proof of concept, we directly detect pesticide dissolved in milk without pretreatment. This detection is enabled by the selective permeation of target molecules into the 3D mesh of the hydrogel skin and the adsorption onto plasmonic hotspots, accompanied by the rejection of large adhesive proteins and colloids. The high sensitivity of nanogap-rich plasmonic nanostructures in a conjunction with the molecular selection of the hydrogel skin enables the fast and reliable detection of tricyclazole in whole milk with a limit of detection as low as 10 ppb within 1 h. We believe that this plasmonic platform is highly adaptable for in situ and on-site detection of small molecules in various complex mixtures including foods, biological fluids, and environmental fluids.

Microfluidic Designing Microgels Containing Highly Concentrated Gold Nanoparticles for SERS Analysis of Complex Fluids

Surface-enhanced Raman scattering (SERS) is one of the most promising methods to detect small molecules for point-of-care analysis as it is rapid, nondestructive, label-free, and applicable for aqueous samples. Here, microgels containing highly concentrated yet evenly dispersed gold nanoparticles are designed to provide SERS substrates that simultaneously achieve contamination-free metal surfaces and high signal enhancement and reproducibility. With capillary microfluidic devices, water-in-oil-in-water (W/O/W) double-emulsion drops are prepared to contain gold nanoparticles and hydrogel precursors in innermost drop. Under hypertonic condition, water is selectively pumped out from the innermost drops. Therefore, gold nanoparticles are gently concentrated without forming aggregates, which are then captured by hydrogel matrix. The resulting microgels have a concentration of gold nanoparticles ≈30 times higher and show Raman intensity two orders of magnitude higher than those with no enrichment. In addition, even distribution of gold nanoparticles results in uniform Raman intensity, providing high signal reproducibility. Moreover, as the matrix of the microgel serves as a molecular filter, large adhesive proteins are rejected, which enables the direct detection of small molecules dissolved in the protein solution. It is believed that this advanced SERS platform is useful for in situ detection of toxic molecules in complex mixtures such as biological fluids, foods, and cosmetics.

Rapid SERS-based recognition of cell secretome on the folic acid-functionalized gold gratings

Nowadays, functionalization of the plasmon-supported nanostructured surface is considered as a powerful tool for tumour cell recognition. In this study, the SERS on a surface plasmon polariton-supported gold grating functionalized with folic acid was used to demonstrate an unpretentious recognition of melanoma-associated fibroblasts. Using cultivation media conditioned by different cells, we were able to detect reproducible differences in the secretome of melanoma-associated and normal control fibroblasts. The homogeneous distribution of plasmon energy along the grating surface was proved to provide excellent SERS signal reproducibility, while, to increase the affinity of (bio)molecules to SERS substrate, folic acid molecules were covalently grafted to the gold gratings. As proof of concept, fibroblasts were cultured in vitro, and culture media from the normal and tumour-associated lines were collected and analysed with our proposed SERS substrates. Identifying individual peaks of the Raman spectra as well as comparing their relative intensities, we showed that the proposed functional SERS platform can recognise the melanoma-associated cells without the need for further statistical spectral evaluation directly. We also demonstrated that the SERS chip created provided a stable SERS signal over a period of 90 days without loss of sensitivity. Graphical abstract.

Building Functional Nanodevices with Vesicle-Templated Porous Polymer Nanocapsules

Vesicle-templated nanocapsules offer a unique combination of properties enabled by robust shells with single-nanometer thickness containing programmed uniform pores capable of fast and selective mass transfer. These capsules emerged as a versatile platform for creating functional devices, such as nanoreactors, nanosensors, and containers for the delivery of drugs and imaging agents. Nanocapsules are synthesized by a directed assembly method using self-assembled bilayers of vesicles as temporary scaffolds. In this approach, hydrophobic building blocks are loaded into the hydrophobic interior of vesicles formed from lipids or surfactants. Pore-forming templates are codissolved with the monomers and cross-linkers in the interior of the bilayer. The polymerization forms a cross-linked shell with embedded pore-forming templates. Removal of the surfactant scaffold and pore-forming templates leads to free-standing nanocapsules with shells containing uniform imprinted nanopores. Development of reliable and scalable synthetic methods for the modular construction of capsules with tunable properties has opened the opportunity to pursue practical applications of nanocapsules. In this Account, we discuss how unique properties of vesicle-templated nanocapsules translate into the creation of functional nanodevices. Specifically, we focus the conversation on applications aiming at the delivery of drugs and imaging agents, creation of fast-acting and selective nanoreactors, and fabrication of nanoprobes for sensing and imaging. We present a brief overview of the synthesis of nanocapsules with an emphasis on recent developments leading to robust synthetic methods including the synthesis under physiological conditions and creation of biodegradable nanocapsules. We then highlight unique properties of nanocapsules essential for practical applications, such as precise control of pore size and chemical environment, selective permeability, and ultrafast transport through the pores. We discuss new motifs for catch and release of small molecules with porous nanocapsules based on controlling the microenvironment inside the nanocapsules, regulating the charge on the orifice of nanopores in the shells, and reversible synergistic action of host and guest forming a supramolecular complex in nanocapsules. We demonstrate successful creation of fast-acting and selective nanoreactors by encapsulation of diverse homogeneous and nanoparticle catalysts. Due to unhindered flow of substrates and products through the nanopores, encapsulation did not compromise catalytic efficiency and, in fact, improved the stability of entrapped catalysts. We present robust nanoprobes based on nanocapsules with entrapped sensing agents and show how the encapsulation resulted in selective measurements with fast response times in challenging conditions, such as small volumes and complex mixtures. Throughout this Account, we highlight the advantages of encapsulation and discuss the opportunities for future design of nanodevices.

Surface-Enhanced Raman Scattering Spectroscopy for Label-Free Analysis of P. aeruginosa Quorum Sensing

Bacterial quorum sensing systems regulate the production of an ample variety of bioactive extracellular compounds that are involved in interspecies microbial interactions and in the interplay between the microbes and their hosts. The development of new approaches for enabling chemical detection of such cellular activities is important in order to gain new insight into their function and biological significance. In recent years, surface-enhanced Raman scattering (SERS) spectroscopy has emerged as an ultrasensitive analytical tool employing rationally designed plasmonic nanostructured substrates. This review highlights recent advances of SERS spectroscopy for label-free detection and imaging of quorum sensing-regulated processes in the human opportunistic pathogen Pseudomonas aeruginosa. We also briefly describe the challenges and limitations of the technique and conclude with a summary of future prospects for the field.

Unraveling the Single-Nanometer Thickness of Shells of Vesicle-Templated Polymer Nanocapsules

Vesicle-templated nanocapsules have emerged as a viable platform for diverse applications. Shell thickness is a critical structural parameter of nanocapsules, where the shell plays a crucial role providing mechanical stability and control of permeability. Here we used small-angle neutron scattering (SANS) to determine the thickness of freestanding and surfactant-stabilized nanocapsules. Despite being at the edge of detectability, we were able to show the polymer shell thickness to be typically 1.0 ± 0.1 nm, which places vesicle-templated nanocapsules among the thinnest materials ever created. The extreme thinness of the shells has implications for several areas: mass-transport through nanopores is relatively unimpeded; pore-forming molecules are not limited to those spanning the entire bilayer; the internal volume of the capsules is maximized; and insight has been gained on how polymerization occurs in the confined geometry of a bilayer scaffold, being predominantly located at the phase-separated layer of monomers and cross-linkers between the surfactant leaflets.

Near infrared surface-enhanced Raman scattering based on star-shaped gold/silver nanoparticles and hyperbolic metamaterial

It is desirable to extend the surface-enhanced Raman scattering (SERS) from the conventionally used visible range into the infrared region, because the fluorescence background is lower in the long-wavelength regime. To do this, it is important to have a SERS substrate suitable for infrared operation. In this work, we report the near infrared SERS operation based on the substrates employing star-shaped gold/silver nanoparticles and hyperbolic metamaterial (HMM) structure. We first fabricate the SERS substrate in which nanoparticles are separated from a silver film by a thin dielectric layer. Performance of the SERS substrate is investigated with a 1064-nm excitation source. Compared with similar silver film-based substrates employing respectively gold and silver spherical nanoparticles, it is found that, Raman intensity scattered by the substrate with star-shaped nanoparticles is 7.4 times stronger than that with gold nanoparticles, and 3.4 times stronger than that with silver nanoparticles. Following this, we fabricate the SERS substrate where the star-shaped nanoparticles are deposited over a HMM structure. The HMM structure comprises three pairs of germanium-silver multilayers. Further experimental result shows that, with the star-shaped nanoparticles, the HMM-based substrate yields 30% higher Raman intensity for near infrared SERS operation than the silver film-based substrate does.

Uniform Microgels Containing Agglomerates of Silver Nanocubes for Molecular Size-Selectivity and High SERS Activity

Surface-enhanced Raman scattering (SERS) is a promising technique for molecular analysis as the molecular fingerprints (Raman spectra) are amplified to detectable levels compared with common spectroscopy. Metal nanostructures localize electromagnetic field on their surfaces, which can lead to dramatic increase of Raman intensity of molecules adsorbed. However, the metal surfaces are prone to contamination, thereby requiring pretreatment of samples to remove adhesive molecules. To avoid the pretreatment and potentially achieve point-of-care (POC) analysis, we have developed SERS-active microgels using the droplet-microfluidic system. As the microgels are composed of water-swollen network with consistent mesh size, they selectively allow diffusion of molecules smaller than the mesh, thereby excluding large adhesives. To render the microgels highly SERS-active, we destabilize silver nanocubes to form agglomerates, which are embedded in the matrix of microgels. The nanogaps in the agglomerates provide high sensitivity in Raman measurement and size-selective permeability of the microgel matrix obviates the pretreatment of samples. To validate the functions, we demonstrate the direct detection of Aspirin dissolved in whole blood without any pretreatment.