Gold nanoparticles reduced in situ and dispersed in polymer thin films: optical and thermal properties

Kinetically controlled metal-elastomer nanophases for environmentally resilient stretchable electronics

Nanophase mixtures, leveraging the complementary strengths of each component, are vital for composites to overcome limitations posed by single elemental materials. Among these, metal-elastomer nanophases are particularly important, holding various practical applications for stretchable electronics. However, the methodology and understanding of nanophase mixing metals and elastomers are limited due to difficulties in blending caused by thermodynamic incompatibility. Here, we present a controlled method using kinetics to mix metal atoms with elastomeric chains on the nanoscale. We find that the chain migration flux and metal deposition rate are key factors, allowing the formation of reticular nanophases when kinetically in-phase. Moreover, we observe spontaneous structural evolution, resulting in gyrified structures akin to the human brain. The hybridized gyrified reticular nanophases exhibit strain-invariant metallic electrical conductivity up to 156% areal strain, unparalleled durability in organic solvents and aqueous environments with pH 2-13, and high mechanical robustness, a prerequisite for environmentally resilient devices.

Transfer Printing of Ordered Plasmonic Nanoparticles at Hard and Soft Interfaces with Increased Fidelity and Biocompatibility Supports a Surface Lattice Resonance

Transfer printing, the relocation of structures assembled on one surface to a different substrate by adjusting adhesive forces at the surface-substrate interface, is widely used to print electronic circuits on biological substrates like human skin and plant leaves. The fidelity of original structures must be preserved to maintain the functionality of transfer-printed circuits. This work developed new biocompatible methods to transfer a nanoscale square lattice of plasmon resonant nanoparticles from a lithographed surface onto leaf and glass substrates. The fidelity of the ordered nanoparticles was preserved across a large area in order to yield, for the first time, an optical surface lattice resonance on glass substrates. To effect the transfer, interfacial adhesion was adjusted by using laser induction of plasmons or unmounted adhesive. Optical and confocal laser scanning microscopy showed that submicron spacing of the square lattice was preserved in ≥90% of transfer-printed areas up to 4 mm2. Up to 90% of ordered nanoparticles were transferred, yielding a surface lattice resonance measured by transmission UV-vis spectroscopy.

Photodynamic toluidine blue-gold nanoconjugates as a novel therapeutic for Staphylococcal biofilms

Staphylococci are among the most frequent bacteria known to cause biofilm-related infections. Pathogenic biofilms represent a global healthcare challenge due to their high tolerance to antimicrobials. In this study, water soluble polyethylene glycol (PEG)-coated gold nanospheres (28 ppm) and nanostars (15 ppm) with electrostatically adsorbed photosensitizer (PS) Toluidine Blue O (TBO) ∼4 μM were successfully synthesized and characterized as PEG-GNPs@TBO and PEG-GNSs@TBO. Both nanoconjugates and the TBO 4 μM solution showed remarkable, if similar, antimicrobial photodynamic inactivation (aPDI) effects at 638 nm, inhibiting the formation of biofilms by two Staphylococcal strains: a clinical methicillin-resistant Staphylococcus aureus (MRSA) isolate and Staphylococcus epidermidis (S. epidermidis) RP62A. Alternatively in biofilm eradication treatments, the aPDI effects of PEG-GNSs@TBO were more effective and yielded a 75% and 50% reduction in viable count of MRSA and S. epidermidis RP62A preformed biofilms, respectively and when compared with untreated samples. This reduction in viable count was even greater than that obtained through aPDI treatment using a 40 μM TBO solution. Confocal laser microscopy (CLSM) and scanning electron microscope (SEM) images of PEG-GNSs@TBO's aPDI treatments revealed significant changes in the integrity and morphology of biofilms, with fewer colony masses. The generation of reactive oxygen species (ROS) upon PEG-GNSs@TBO's aPDI treatment was detected by CLSM using a specific ROS fluorescent probe, demonstrating bright fluorescence red spots across the surfaces of the treated biofilms. Our findings shine a light on the potential synergism between gold nanoparticles (AuNPs) and photosensitizers in developing novel nanoplatforms to target Staphylococcal biofilm related infections.

Tailoring the Barrier Properties of PLA: A State-of-the-Art Review for Food Packaging Applications

It is now well recognized that the production of petroleum-based packaging materials has created serious ecological problems for the environment due to their resistance to biodegradation. In this context, substantial research efforts have been made to promote the use of biodegradable films as sustainable alternatives to conventionally used packaging materials. Among several biopolymers, poly(lactide) (PLA) has found early application in the food industry thanks to its promising properties and is currently one of the most industrially produced bioplastics. However, more efforts are needed to enhance its performance and expand its applicability in this field, as packaging materials need to meet precise functional requirements such as suitable thermal, mechanical, and gas barrier properties. In particular, improving the mass transfer properties of materials to water vapor, oxygen, and/or carbon dioxide plays a very important role in maintaining food quality and safety, as the rate of typical food degradation reactions (i.e., oxidation, microbial development, and physical reactions) can be greatly reduced. Since most reviews dealing with the properties of PLA have mainly focused on strategies to improve its thermal and mechanical properties, this work aims to review relevant strategies to tailor the barrier properties of PLA-based materials, with the ultimate goal of providing a general guide for the design of PLA-based packaging materials with the desired mass transfer properties.

Gold Nanostars Embedded in PDMS Films: A Photothermal Material for Antibacterial Applications

Bacteria infections and related biofilms growth on surfaces of medical devices are a serious threat to human health. Controlled hyperthermia caused by photothermal effects can be used to kill bacteria and counteract biofilms formation. Embedding of plasmonic nano-objects like gold nanostars (GNS), able to give an intense photothermal effect when irradiated in the NIR, can be a smart way to functionalize a transparent and biocompatible material like polydimethylsiloxane (PDMS). This process enables bacteria destruction on surfaces of PDMS-made medical surfaces, an action which, in principle, can also be exploited in subcutaneous devices. We prepared stable and reproducible thin PDMS films containing controllable quantities of GNS, enabling a temperature increase that can reach more than 40 degrees. The hyperthermia exerted by this hybrid material generates an effective thermal microbicidal effect, killing bacteria with a near infrared (NIR) laser source with irradiance values that are safe for skin.

Cell-surface interactions on gold-coated polydimethylsiloxane nanocomposite structures: Localized laser heating on cell viability

This article presents the results of cell-surface interactions on polydimethylsiloxane (PDMS)-based substrates coated with nanoscale gold (Au) thin films. The surfaces of PDMS and PDMS-magnetite (MNP)-based substrates were treated with UV-ozone, prior to thermal vapor deposition (sputter-coated) of thin films of titanium (Ti) onto the substrates to improve the adhesion of Au coatings. The thin layer of Ti was thermally evaporated to improve interfacial adhesion, which was enhanced by a 40-nm thick film microwrinkled/buckled wavy layer of Au, that was coated to enhance cell-surface interactions and protein absorption. Cell-surface interactions were studied on the hybrid surfaces using a combination of optical and fluorescence microscopy. Consequently, cell proliferation and surface cytotoxicity (of the sputter-coated PDMS surfaces) were elucidated by characterizing the metabolic activity in the presence of breast cancer and normal breast cells. The photothermal conversion efficiency associated with laser-materials interactions with the PDMS/PDMS-magnetite-based composites was shown to have an optimum efficiency of ~31.8%. The implications of the results are discussed for potential applications of PDMS nanocomposites in implantable biomedical devices.

Effects of geometry and composition of soft polymer films embedded with nanoparticles on rates for optothermal heat dissipation

Embedding soft matter with nanoparticles (NPs) can provide electromagnetic tunability at sub-micron scales for a growing number of applications in healthcare, sustainable energy, and chemical processing. However, the use of NP-embedded soft material in temperature-sensitive applications has been constrained by difficulties in validating the prediction of rates for energy dissipation from thermally insulating to conducting behavior. This work improved the embedment of monodisperse NPs to stably decrease the inter-NP spacings in polydimethylsiloxane (PDMS) to nano-scale distances. Lumped-parameter and finite element analyses were refined to apportion the effects of the structure and composition of the NP-embedded soft polymer on the rates for conductive, convective, and radiative heat dissipation. These advances allowed for the rational selection of PDMS size and NP composition to optimize measured rates of internal (conductive) and external (convective and radiative) heat dissipation. Stably reducing the distance between monodisperse NPs to nano-scale intervals increased the overall heat dissipation rate by up to 29%. Refined fabrication of NP-embedded polymer enabled the tunability of the dynamic thermal response (the ratio of internal to external dissipation rate) by a factor of 3.1 to achieve a value of 0.091, the largest reported to date. Heat dissipation rates simulated a priori were consistent with 130 μm resolution thermal images across 2- to 15-fold changes in the geometry and composition of NP-PDMS. The Nusselt number was observed to increase with the fourth root of the Rayleigh number across thermally insulative and conductive regimes, further validating the approach. These developments support the model-informed design of soft media embedded with nano-scale-spaced NPs to optimize the heat dissipation rates for evolving temperature-sensitive diagnostic and therapeutic modalities, as well as emerging uses in flexible bioelectronics, cell and tissue culture, and solar-thermal heating.

Design and Near-Infrared Actuation of a Gold Nanorod⁻Polymer Microelectromechanical Device for On-Demand Drug Delivery

Polymeric drug delivery systems usually deliver drugs by diffusion with an initial burst of release followed by a slower prolonged release phase. An optimal system would release exact doses of drugs using an on-demand external actuation system. The purpose of this study was to design and characterize a novel drug-delivery device that utilizes near infrared (NIR 800 nm) laser-actuated drug release. The device was constructed from biocompatible polymers comprising a reservoir of drug covered by an elastic perforated diaphragm composed of a bilayer of two polymers with different thermal expansion coefficients (ethylenevinylacetate (EVA) and polydimethylsiloxane (PDMS) containing gold nanoparticles). Upon illumination with a NIR laser, the gold nanoparticles rapidly heated the bilayer resulting in bending and a drug-pumping action through the perforated bilayer, following sequential laser-actuation cycles. Devices filled with the anti-proliferative drug docetaxel were seen to release only small amounts of drug by diffusion but to release large and reproducible amounts of drug over 20 s laser-actuation periods. Because NIR 800 nm is tissue-penetrating without heating tissue, suitable geometry drug-delivery devices might be implanted in the body to be actuated by an externally applied NIR laser to allow for on-demand exact drug dosing in vivo.

Encapsulation of Single Nanoparticle in Fast-Evaporating Micro-droplets Prevents Particle Agglomeration in Nanocomposites

This work describes the use of fast-evaporating micro-droplets to finely disperse nanoparticles (NPs) in a polymer matrix for the fabrication of nanocomposites. Agglomeration of particles is a key obstacle for broad applications of nanocomposites. The classical approach to ensure the dispersibility of NPs is to modify the surface chemistry of NPs with ligands. The surface properties of NPs are inevitably altered, however. To overcome the trade-off between dispersibility and surface-functionality of NPs, we develop a new approach by dispersing NPs in a volatile solvent, followed by mixing with uncured polymer precursors to form micro-droplet emulsions. Most of these micro-droplets contain no more than one NP per drop, and they evaporate rapidly to prevent the agglomeration of NPs during the polymer curing process. As a proof of concept, we demonstrate the design and fabrication of TiO₂ NP@PDMS nanocomposites for solar fuel generation reactions with high photocatalytic efficiency and recyclability arising from the fine dispersion of TiO₂. Our simple method eliminates the need for surface functionalization of NPs. Our approach is applicable to prepare nanocomposites comprising a wide range of polymers embedded with NPs of different composition, sizes, and shapes. It has the potential for creating nanocomposites with novel functions.

Thermoplasmonic dissipation in gold nanoparticle–polyvinylpyrrolidone thin films

Dissipated heat was consistent with power extinguished by absorbing nanoparticles dispersed into thin polymer films at subwavelength intervals. Measurements mirrored a priori simulation of optical and thermal responses. Components of heating and absorption were identified.

Approach and Coalescence of Gold Nanoparticles Driven by Surface Thermodynamic Fluctuations and Atomic Interaction Forces

The approach and coalescence behavior of gold nanoparticles on a silicon surface were investigated by experiments and molecular dynamics simulations. By analyzing the behavior of the atoms in the nanoparticles in the simulations, it was found that the atoms in a single isolated nanoparticle randomly fluctuated and that the surface atoms showed greater fluctuation. The fluctuation increased as the temperature increased. When there were two or more neighboring nanoparticles, the fluctuating surface atoms of the nanoparticles "flowed" toward the neighboring nanoparticle because of atomic interaction forces between the nanoparticles. With the surface atoms "flowing", the gold nanoparticles approached and finally coalesced. The simulation results were in good agreement with the experimental results. It can be concluded that surface thermodynamic fluctuations and atomic interaction forces are the causes of the approach and coalescence behavior of the gold nanoparticles.

The unusual visible photothermal response of free standing multilayered films based on plasmonic bimetallic nanocages

An unusual photothermal response in the visible region has been observed in free standing multilayered films based on the plasmonic bimetallic Au and Ag nanocages (Ag@AuNCs).

Role of Surfactant in the Formation of Gold Nanoparticles in Aqueous Medium

The stability of gold nanoparticles is a major issue which decides their impending usage in nanobiotechnological applications. Often biomimetically synthesized nanoparticles are deemed useless owing to their instability in aqueous medium. So, surfactants are used to stabilize the nanoparticles. But does the surfactant only stabilize by being adsorbed to the surface of the nanoparticles and not play significantly in moulding the size and shape of the nanoparticles? Keeping this idea in mind, gold nanoparticles (GNPs) synthesized by l-tryptophan (Trp) mediated reduction of chloroauric acid (HAuCl4) were stabilized by anionic surfactant, sodium dodecyl sulphate (SDS), and its effect on the moulding of size and properties of the GNPs was studied. Interestingly, unlike most of the gold nanoparticles synthesis mechanism showing saturation growth mechanism, inclusion of SDS in the reaction mixture for GNPs synthesis resulted in a bimodal mechanism which was studied by UV-Vis spectroscopy. The mechanism was further substantiated with transmission electron microscopy. Zeta potential of GNPs solutions was measured to corroborate stability observations recorded visually.

Photothermal response of the plasmonic nanoconglomerates in films assembled by electroless plating

Conversion of light energy to heat by ordered gold nanostructures on a gold film has been investigated.

Asymmetric reduction of gold nanoparticles into thermoplasmonic polydimethylsiloxane thin films

Polymer thin films containing gold nanoparticles (AuNPs) are of growing interest in photovoltaics, biomedicine, optics, and nanoelectromechanical systems (NEMs). This work has identified conditions to rapidly reduce aqueous hydrogen tetrachloroaurate (TCA) that is diffusing into one exposed interface of a partially cured polydimethylsiloxane (PDMS) thin film into AuNPs. Nanospheroids, irregular gold (Au) networks, and micrometer-sized Au conglomerates were formed in a ∼5 μm layer at dissolved TCA contents of 0.005, 0.05, and 0.5 mass percent, respectively. Multiscale morphological, optical, and thermal properties of the resulting asymmetric AuNP-PDMS thin films were characterized. Reduction of TCA diffusing into the interface of partially cured PDMS film increased AuNP content, robustness, and scalability relative to laminar preparation of asymmetric AuNP-PDMS thin films. Optical attenuation and thermoplasmonic film temperature due to incident resonant irradiation increased in linear proportion to the order of magnitude increases in TCA content, from 0.005 to 0.05 to 0.5 mass percent. At the highest TCA content (0.05 mass percent), an asymmetric PDMS film 52-μm-thick with a 7 μm AuNP-containing layer was produced. It attenuated 85% of 18 mW of incident radiation and raised the local temperature to 54.5 °C above ambient. This represented an increase of 3 to 230-fold in photon-to-heat efficiency over previous thermoplasmonic AuNP-containing systems.

Optical attenuation of plasmonic nanocomposites within photonic devices

Plasmonic nanostructures enable microscopic optical manipulation such as light trapping in photonic devices. However, integration of embedded nanostructures into photonic devices has been limited by tractability of nanoscale and microscale descriptions in device architectures. This work uses a linear algebraic model to distinguish geometric optical responses of nanoparticles integrated into dielectric substrates interacting with macroscopic back-reflectors from absorptive and nonlinear plasmonic effects. Measured transmission, reflection, and attenuation (losses) from ceramic and polymer composites supporting two- and three-dimensional distributions of gold nanoparticles, respectively, are predictable using the model. A unique equilateral display format correlates geometric optical behavior and attenuation to nanoparticle density and back-reflector opacity, allowing intuitive, visual specification of density and opacity necessary to obtain a particular optical performance. The model and display format are useful for facile design and integration of plasmonic nanostructures into photonic devices for light manipulation.