Thermoresponsive plasmonic core-satellite nanostructures with reversible, temperature sensitive optical properties

Poly(ionic liquid)s-Based Thermal-Responsive Microgel for Use as SERS Substrates with "ON-OFF" Switchable Effect

A temperature-responsive surface-enhanced Raman scattering (SERS) substrate with "ON-OFF" switching based on poly(ionic liquid)s (PILs) block copolymer microgels have been designed and synthesized. The PIL units act as a joint component to anchor the gold nanoparticles (AuNPs) and analytes onto poly(N-isopropylacrylamide) (PNIPAm). This anchor allows the analytes to be fixed at the formed hot spots under temperature stimulus. Owing to the regulation of the PNIPAm segment, the SERS substrates exhibit excellent thermally responsive SERS activity with a reversible "ON-OFF" effect. Additionally, because of the anion exchange of PILs, microgels can introduce new analytes, which offers more flexibility for the system. The substrate shows excellent reversibility, controllability, and flexibility of SERS activity, which is expected to have a broad application in the field of practical SERS sensors.

Kinetically Controlled Site-Specific Self-assembly of Hairy Colloids

The solvophobicity-driven directional self-assembly of polymer-coated gold nanorods is a well-established phenomenon. Yet, the kinetics of this process, the origin of site-selectivity in the self-assembly, and the interplay of (attractive) solvophobic brush interactions and (repulsive) electrostatic forces are not fully understood. Herein, we use a combination of time-resolved (vis/NIR) extinction spectroscopy and finite-difference time-domain (FDTD) simulations to determine conversion profiles for the assembly of gold nanorods with polystyrene shells of distinct thicknesses into their (tip-to-tip) self-assembled structures. In particular, we demonstrate that the assembly process is highly protracted compared with diffusion-controlled rates, and we find that the assembly rate varies for different thickness values of the polymer shell. Our findings were rationalized using coarse-grained molecular dynamics simulations, which also corroborated the tip-to-tip preference in the self-assembly process, albeit with a uniform polymer coating. Utilizing the knowledge of quantified conversion rates for distinct colloidal species, we designed coassembling systems with different brush thicknesses, featuring "narcissistic" self-sorting behavior. This provides new perspectives for high-level supracolloidal self-assembly.

Turning on hotspots: supracolloidal SERS probes made brilliant by an external activation mechanism

We achieved external activation of local hot-spot sites in supracolloidal assembly structures. The concept was demonstrated by boosting surface-enhanced Raman scattering (SERS) efficiency by one order of magnitude through a heating-induced process. Our approach involves assembling gold nanoparticles with distinct dimensions, i.e. 16 and 80 nm, into well-defined planet-satellite-type arrangement structures using thermoresponsive (poly(N-isopropylacrylamide)) star polymer linkers. Insights into the assembly process were obtained by calculations within the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory framework. We observe one order of magnitude increase in SERS enhancement by a heating-induced volume-phase transition. This magnification aligns with simulations run using the finite-difference time-domain (FDTD) method. The implications of this adaptive supracolloidal concept are twofold: Firstly, our approach bypasses limitations of existing systems that are associated with the limited accessibility of electromagnetic hot-spot sites in strongly coupled, static assemblies of plasmonic nanoparticles, by providing the capability of dynamic hot-spot re-configuration. Second, these externally activated probes offer promising opportunities for the development of messenger materials and associated sensing strategies.

Nano-engineering nanomedicines with customized functions for tumor treatment applications

Nano-engineering with unique "custom function" capability has shown great potential in solving technical difficulties of nanomaterials in tumor treatment. Through tuning the size and surface properties controllablly, nanoparticles can be endoewd with tailored structure, and then the characteristic functions to improve the therapeutic effect of nanomedicines. Based on nano-engineering, many have been carried out to advance nano-engineering nanomedicine. In this review, the main research related to cancer therapy attached to the development of nanoengineering nanomedicines has been presented as follows. Firstly, therapeutic agents that target to tumor area can exert the therapeutic effect effectively. Secondly, drug resistance of tumor cells can be overcome to enhance the efficacy. Thirdly, remodeling the immunosuppressive microenvironment makes the therapeutic agents work with the autoimmune system to eliminate the primary tumor and then prevent tumor recurrence and metastasis. Finally, the development prospects of nano-engineering nanomedicine are also outlined.

Stimuli-responsive plasmonic core-satellite hybrid nanostructures with tunable nanogaps

Incorporating stimuli-responsive block copolymers to hierarchical metallic nanoparticles (MNPs) is of particular interest due to their tunable plasmonic properties responding to environmental stimuli. We herein report thermo-responsive plasmonic core-satellite hybrid nanostructures with tunable nanogaps as surface-enhanced Raman scattering (SERS) nanotags. Two different diblock copolymers with opposite charges, poly(acrylic acid-b-N-isopropylacrylamide) (p(AAc-b-NIPAM)) and poly(N,N-dimethylaminoethyl methacrylate-b-N-isopropylacrylamide) (p(DMAEMA-b-NIPAM)), were synthesized. The negatively charged p(AAc-b-NIPAM)s were bound to gold nanospheres (GNSs), while the positively charged p(DMAEMA-b-NIPAM)s were conjugated to gold nanorods (GNRs) via gold-sulfur bonds. When p(AAc-b-NIPAM)-GNSs and p(DMAEMA-b-NIPAM)-GNRs were electrostatically complexed, plasmonic hybrid nanostructures consisting of both GNS satellites and a GNR core were formed. Dynamic tuning of electromagnetic coupling of their nanogaps was achieved via a temperature-triggered conformational change of p(NIPAM) blocks. Furthermore, a sandwich-type immunoassay for the detection of immunoglobulin G was performed to demonstrate these core-satellites as potential SERS nanotags. Our results showed that these plasmonic core-satellites with stimuli-responsiveness are promising for SERS-based biosensing applications.

Universal linker-free assembly of core-satellite hetero-superstructures

Colloidal superstructures comprising hetero-building blocks often show unanticipated physical and chemical properties. Here, we present a universal assembly methodology to prepare hetero-superstructures. This straightforward methodology allows the assembly of building block materials varying from inorganic nanoparticles to living cells to form superstructures. No molecular linker is required to bind the building blocks together and thus the products do not contain any unwanted adscititious material. The Fourier transform infrared spectra, high resolution transmission electron microscopic images and nanoparticle adhesion force measurement results reveal that the key to self-organization is stripping surface ligands by adding non-polar solvents or neutralizing surface charge by adding salts, which allow us to tune the balance between van der Waals attraction and electrostatic repulsion in the colloid so as to trigger the assembling process. As a proof-of-concept, the superior photocatalytic activity and single-particle surface-enhanced Raman scattering of the corresponding superstructures are demonstrated. Our methodology greatly extends the scope of building blocks for superstructure assembly and enables scalable construction of colloidal multifunctional materials.

Assembly of partially covered strawberry supracolloids in dilute and concentrate aqueous dispersions

HYPOTHESIS

Core-corona supracolloids can be assembled in aqueous dispersions by controlling the physical interactions between the corona and core colloidal particles. A raspberry corona configuration with full surface coverage of the core can be reached by inducing strong attractive interactions between the individual particles. A controlled partial surface coverage of the core, i.e. strawberry configuration, is however, more difficult to achieve. Supracolloids with different surface coverage ratio exhibit unique and multifunctional surface properties.

EXPERIMENTS

By counterbalancing the multiple physical interactions playing a role during the assembly, the configuration and stability of the assemblies could be fine-tuned over a wide range of concentrations. Supracolloids consisting of polyethylene glycol (PEO)-grafted polymer particles covered by silica nanoparticles were assembled with different configurations, by adjusting the pH and ionic strength of the dispersion, the PEO grafting density and the particles concentration. The self-assembly process and resulting configurations were monitored via cryogenic transmission electron microscopy (Cryo-TEM) and light scattering.

FINDINGS

The suitable conditions to assemble supracolloids with partial corona coverage have been established. Stable strawberry supracolloids could be prepared, both for diluted (1 wt%) and concentrated (12 wt%) dispersions. These hybrid supracolloids with well-defined configuration are highly relevant to developing advanced water-borne paints and inks, food dispersions, cosmetic and healthcare products.

Materials with Tunable Optical Properties for Wearable Epidermal Sensing in Health Monitoring

Advances in wearable epidermal sensors have revolutionized the way that physiological signals are captured and measured for health monitoring. One major challenge is to convert physiological signals to easily readable signals in a convenient way. One possibility for wearable epidermal sensors is based on visible readouts. There are a range of materials whose optical properties can be tuned by parameters such as temperature, pH, light, and electric fields. Herein, this review covers and highlights a set of materials with tunable optical properties and their integration into wearable epidermal sensors for health monitoring. Specifically, the recent progress, fabrication, and applications of these materials for wearable epidermal sensors are summarized and discussed. Finally, the challenges and perspectives for the next generation wearable devices are proposed.

Synthesis of SERS-active core-satellite nanoparticles using heterobifunctional PEG linkers

Surface-enhanced Raman scattering (SERS) is a sensitive analytical technique capable of magnifying the vibrational intensity of molecules adsorbed onto the surface of metallic nanostructures. Various solution-based SERS-active metallic nanostructures have been designed to generate substantial SERS signal enhancements. However, most of these SERS substrates rely on the chemical aggregation of metallic nanostructures to create strong signals. While this can induce high SERS intensities through plasmonic coupling, most chemically aggregated assemblies suffer from poor signal reproducibility and reduced long-term stability. To overcome these issues, here we report for the first time the synthesis of gold core-satellite nanoparticles (CSNPs) for robust SERS signal generation. The novel CSNP assemblies consist of a 30 nm spherical gold core linked to 18 nm satellite particles via linear heterobifunctional thiol-amine terminated PEG chains. We explore the effects that the varying chain lengths have on SERS hot-spot generation, signal reproducibility and long-term activity. The chain length was varied by using PEGs with different molecular weights (1000 Da, 2000 Da, and 3500 Da). The CSNPs were characterized via UV-Vis spectrophotometry, transmission electron microscopy (TEM), ζ-potential measurements, and lastly SERS measurements. The versatility of the synthesized SERS-active CSNPs was revealed through characterization of optical stability and SERS enhancement at 0, 1, 3, 5, 7 and 14 days.

A combination strategy based on an Au nanorod/doxorubicin gel via mild photothermal therapy combined with antigen-capturing liposomes and anti-PD-L1 agent promote a positive shift in the cancer-immunity cycle

The antitumor immune response involves a cascade of cancer-immunity cycles. Developing a combination therapy aimed at the cancer-immunity cycle is of great importance. In this research, we designed and tested a combined therapeutic-Au nanorod (AuNR)/doxorubicin (DOX) gel (AuNR/DOX gel)-in which the sustained release of DOX was controlled by Pluronic gel. DOX served as an immunogenic tumor cell death (ICD) inducer, triggering the production of damage-associated molecular patterns (DAMPs). Mild photothermal therapy (Mild PTT) produced by 880 nm laser-irradiated AuNRs also generated tumor-associated antigens. Maleimide-modified liposomes (L-Mals), as antigen capturing agents, promoted tumor antigen uptake by DCs. Ultimately, more CD8+ T cells and fewer regulatory T cells (Tregs) infiltrated the tumor, eliciting antitumor responses from the PD-L1 antibody. Our results indicate that this combination strategy promotes a positive shift in the cancer-immunity cycle and holds much promise for combination strategy will lead to development of an antitumor drug delivery system. STATEMENT OF SIGNIFICANCE: Developing a combination therapy for cancer-immunity cycle is of great importance due to antitumor immune response involving a cascade of cancer-immunity cycles. Cancer-immunity cycle usually includes tumor antigen release, antigen presentation, immune activation, trafficking, infiltration, specific recognition of tumor cells by T cells, and finally cancer cell killing. In this research, we designed a combination strategy based on Au nanorod/doxorubicin gel via mild photothermal therapy combined with antigen-capturing liposomes and anti-PD-L1 agent promoting a positive shift in the cancer-immunity cycle. Our results indicate that this combination strategy promotes a positive shift in the cancer-immunity cycle and holds much promise for combination strategy will lead to development of an antitumor drug delivery system.

Constrained thermoresponsive polymers - new insights into fundamentals and applications

In the last decades, numerous stimuli-responsive polymers have been developed and investigated regarding their switching properties. In particular, thermoresponsive polymers, which form a miscibility gap with the ambient solvent with a lower or upper critical demixing point depending on the temperature, have been intensively studied in solution. For the application of such polymers in novel sensors, drug delivery systems or as multifunctional coatings, they typically have to be transferred into specific arrangements, such as micelles, polymer films or grafted nanoparticles. However, it turns out that the thermodynamic concept for the phase transition of free polymer chains fails, when thermoresponsive polymers are assembled into such sterically confined architectures. Whereas many published studies focus on synthetic aspects as well as individual applications of thermoresponsive polymers, the underlying structure-property relationships governing the thermoresponse of sterically constrained assemblies, are still poorly understood. Furthermore, the clear majority of publications deals with polymers that exhibit a lower critical solution temperature (LCST) behavior, with PNIPAAM as their main representative. In contrast, for polymer arrangements with an upper critical solution temperature (UCST), there is only limited knowledge about preparation, application and precise physical understanding of the phase transition. This review article provides an overview about the current knowledge of thermoresponsive polymers with limited mobility focusing on UCST behavior and the possibilities for influencing their thermoresponsive switching characteristics. It comprises star polymers, micelles as well as polymer chains grafted to flat substrates and particulate inorganic surfaces. The elaboration of the physicochemical interplay between the architecture of the polymer assembly and the resulting thermoresponsive switching behavior will be in the foreground of this consideration.

Nanoengineering with RAFT polymers: from nanocomposite design to applications

Reversible addition–fragmentation chain-transfer (RAFT) polymerization is a powerful tool for the precise formation of macromolecular building blocks that can be used for the construction of well-defined nanocomposites.

Tuning drug dosing through matching optically active polymer composition and NIR stimulation parameters

Controlled release is at the forefront of modern bioscience as it aims to address challenges associated with the dosing of drugs within required levels for therapeutic effect. Many materials and approaches can be used to control the release from different reservoirs including nanoparticles, liposomes and hydrogels. Using thermoresponsive hydrogels, near infrared illumination of plasmonic nanoparticles can be used to control the hydrogel through localised surface plasmon resonance heating. This work extends beyond a material level and pursues detailed examination of the drug release characteristics of a variable acrylic acid poly(N-isopropylacrylamide) coated gold nanorod system using dexamethasone as a model drug. Release was examined under different irradiation power densities and exposure times. Bulk heating effects in all stimulation protocols did not exceed the lower critical solution temperature of the system, but a marked increase in release was seen following stimulation. This was likely due to more intense heating occurring around the nanorods. A release model was established to describe the amount of drug eluted relative to input energy, suggesting that shorter irradiation periods release the drug more efficiently. The data reported establishes plasmonically modulated thermosensitive hydrogels as a candidate material that can be tailored to specific clinical applications of stimulated release.

Electrotunable Nanoplasmonics for Amplified Surface Enhanced Raman Spectroscopy

Tuning the properties of optical metamaterials in real time is one of the grand challenges of photonics. Being able to do so will enable a class of adaptive photonic materials for use in applications such as surface enhanced Raman spectroscopy and reflectors/absorbers. One strategy to achieving this goal is based on the electrovariable self-assembly and disassembly of two-dimensional nanoparticle arrays at a metal | liquid interface. As expected, the structure results in plasmonic coupling between NPs in the array but perhaps as importantly between the array and the metal surface. In such a system, the density of the nanoparticle array can be reversibly controlled by the variation of electrode potential. Theory suggests that due to a collective plasmon-coupling effect  less than 1 V variation of electrode potential can give rise to a dramatic simultaneous change in optical reflectivity from ∼93% to ∼1% and the amplification of the SERS signal by up to 5 orders of magnitude. This is experimentally demonstrated using a platform based on the voltage-controlled assembly of 40 nm Au-nanoparticle arrays at a TiN/Ag electrode in contact with an aqueous electrolyte. We show that all the physics underpinning the behavior of this platform works precisely as suggested by the proposed theory, setting the electrochemical nanoplasmonics as a promising direction in photonics research.

Optical tweezers-based characterisation of gold core-satellite plasmonic nano-assemblies incorporating thermo-responsive polymers

We report on the characterisation of the optical properties and dynamic behaviour of optically trapped single stimuli-responsive plasmonic nanoscale assemblies. Nano-assemblies consist of a core-satellite arrangement where the constituent nanoparticles are connected by the thermoresponsive polymer, poly(DEGA-co-OEGA). The optical tweezers allow the particles to be held isolated in solution and interrogated using dark-field spectroscopy. Additionally, controlling the optical trapping power provides localised heating for probing the thermal response of the nanostructures. Our results identify a number of distinct core-satellite configurations that can be stably trapped, which are verified using finite element modelling. Laser heating of the nanostructures through the trapping laser yields irreversible modification of the arrangement, as observed through the scattering spectrum. We consider which factors may be responsible for the observed behaviour in the context of the core-satellite geometry, polymer-solvent interaction, and the bonding of the nanoparticles.

Toward Flexible Surface-Enhanced Raman Scattering (SERS) Sensors for Point-of-Care Diagnostics

Surface-enhanced Raman scattering (SERS) spectroscopy provides a noninvasive and highly sensitive route for fingerprint and label-free detection of a wide range of molecules. Recently, flexible SERS has attracted increasingly tremendous research interest due to its unique advantages compared to rigid substrate-based SERS. Here, the latest advances in flexible substrate-based SERS diagnostic devices are investigated in-depth. First, the intriguing prospect of point-of-care diagnostics is briefly described, followed by an introduction to the cutting-edge SERS technique. Then, the focus is moved from conventional rigid substrate-based SERS to the emerging flexible SERS technique. The main part of this report highlights the recent three categories of flexible SERS substrates, including actively tunable SERS, swab-sampling strategy, and the in situ SERS detection approach. Furthermore, other promising means of flexible SERS are also introduced. The flexible SERS substrates with low-cost, batch-fabrication, and easy-to-operate characteristics can be integrated into portable Raman spectroscopes for point-of-care diagnostics, which are conceivable to penetrate global markets and households as next-generation wearable sensors in the near future.

Precise Self-Assembly and Controlled Catalysis of Thermoresponsive Core-Satellite Multicomponent Hybrid Nanoparticles

The construction of multicomponent hybrid nanomaterials with well-controlled architecture, especially bearing an ordered homogeneity and distribution of the subunits with tunable functions, is a key challenge in chemistry and material science. Herein, we reported a versatile and novel strategy to fabricate core-satellite multicomponent nanostructures with tunable interparticle distances and catalysis properties by the combination of surface-initiated reversible addition-fragmentation chain transfer (SI-RAFT) polymerization and self-assembly. The arrangement and interparticle distance of gold satellites could be precisely tuned by the SI-RAFT polymerization process and the feeding ratio of gold nanoparticles (AuNPs) and the core nanoparticle. It is worth to note that multilayered core-satellite nanostructures have been fabricated by a high-feeding ratio of AuNPs and magnetite NP (MNP)@SiO₂-PNIPAm. Notably, the core-satellite MNP@SiO₂-PNIPAm-Au nanoparticles exhibited excellent thermoresponsive behaviors with the change of temperature. Furthermore, the catalytic efficiency of MNP@SiO₂-PNIPAm-Au nanoparticles via the reduction of 4-nitrophenol to 4-aminophenol can be well modulated by the nanoparticle size, temperature, and polymer feed ratio. This strategy for precise construction of core-satellite nanostructures would open a new pathway to construct multicomponent functional nanostructures.