Light as a construction tool of metal nanoparticles: Synthesis and mechanism

A rapid green synthesis of Ag/AgCl-NC photocatalyst for environmental applications

The present study focuses on extract-mediated Ag nanoparticles (NPs), AgCl-NPs, and Ag/AgCl nanocomposites (NCs) as photocatalysts along with its antimicrobial and dye degradation activities. The synthesis of these NPs and NCs was performed by using Azadirachta indica plant fruit extract and analyzed using UV-Vis spectroscopy to confirm the synthesis and band gap of these NPs and NCs, X-ray diffraction (XRD) to determine its size and crystalline nature. Fourier transform infrared spectroscopy (FTIR) to discern phytochemicals, responsible for the reduction and capping of the synthesized NCs. Scanning electron microscopy analysis (SEM), transmission electron microscopy analysis (TEM), and energy dispersive X-ray (EDX) spectroscopy analysis were performed to validate the morphology and presence of silver and chloride percentage in the composites. Later, these NPs and NCs were used for their potential role in photocatalytic degradation of methylene blue dye and antibacterial activity against Escherichia coli and Staphylococcus aureus of human pathogen. The prepared Ag/AgCl-NCs exhibited an enhanced photocatalytic and antibacterial activities in comparison with pure Ag and AgCl nanomaterials. However, green-synthesized NPs and NCs played dual roles as a photocatalyst and antibacterial agent in various biomedical and industrial sectors. Moreover, we found that it might be a hot research in many other environmental applications in upcoming days.

A Review on the Synthesis and Functionalization of Gold Nanoparticles as a Drug Delivery Vehicle

Metal nanoparticles are being extensively used in biomedical fields due to their small size-to-volume ratio and extensive thermal stability. Gold nanoparticles (AuNPs) are an obvious choice for biomedical applications due to their amenability of synthesis, stabilization, and functionalization, low toxicity, and ease of detection. In the past few decades, various chemical methods have been used for the synthesis of AuNPs, but recently, newer environment friendly green approaches for the synthesis of AuNPs have gained attention. AuNPs can be conjugated with a number of functionalizing moieties including ligands, therapeutic agents, DNA, amino acids, proteins, peptides, and oligonucleotides. Recently, studies have shown that gold nanoparticles not only infiltrate the blood vessels to reach the site of tumor but also enter inside the organelles, suggesting that they can be employed as effective drug carriers. Moreover, after reaching their target site, gold nanoparticles can release their payload upon an external or internal stimulus. This review focuses on recent advances in various methods of synthesis of AuNPs. In addition, strategies of functionalization and mechanisms of application of AuNPs in drug and bio-macromolecule delivery and release of payloads at target site are comprehensively discussed.

Uptake of silver, gold, and hybrids silver-iron, gold-iron and silver-gold aminolevulinic acid nanoparticles by MCF-7 breast cancer cells

BACKGROUND

Nanoparticles show promise for theranostic applications in cancer. The metal-based nanoparticles can be used both as photosensitizers and delivery vehicles. In bimetallic particles based on gold or silver and iron, a combination of the plasmonic features of the gold or silver components with the magnetic properties of the iron makes these hybrid nanomaterials suitable for both imaging and therapeutic applications. Herein, we discuss toxicity and cell internalization of metallic (silver and gold) and bimetallic (silver-iron, gold-iron, and silver-gold) aminolevulinic acid (ALA) nanoparticles. ALA can control the production of an intracellular photosensitizer, protoporphyrin IX (PpIX), commonly used in photodynamic therapy.

METHODS

Nanoparticles were synthesized by photoreduction method and characterized by UV/Vis spectra, Zeta potential, FTIR, XRD, and transmission electron microscopy. The amount of singlet oxygen generation by a yellow LED, and ultrasound was studied for gold, gold-iron, and silver-gold nanoparticles. Cytotoxicity assays of MCF-7 in the presence of nanoparticles were performed, and PpIX fluorescence was quantified by high content screening (HCS).

RESULTS

Red fluorescence observed after 24 h of nanoparticles incubation on MCF-7 cells, indicated that the ALA in surface of nanoparticles was efficiently converted to PpIX. The best results for singlet oxygen generation with LED or ultrasound irradiation were obtained with ALA:AgAuNPs.

CONCLUSIONS

The studied nanoparticles present the potential to deliver aminolevulinic acid to breast cancer cells efficiently, generate singlet oxygen, and convert ALA into PpIX inside the cells allowing photodiagnosis and therapies such as photodynamic and sonodynamic therapies.

Optical characterization and tunable antibacterial properties of gold nanoparticles with common proteins

The interaction of three proteins, viz. Bovine Serum Albumin (BSA), Human Serum Albumin (HSA) and Hen Egg White Lysozyme (HEWL) with gold nanoparticles (GNPs) is investigated using surface plasmon resonance (SPR) spectroscopy, fluorescence spectroscopy and circular dichroism (CD). Size and morphology of the samples was established using Transmission Electron Microscopy (TEM) and stability studies was established using zeta potential analysis. The stability of protein-GNP complex was found to be greater than that of individual protein as well as individual GNPs. Also HEWL-GNP complex was more stable compared to the other protein complexes. Absorbance of proteins increases with increase in gold nanoparticle concentration due to the extension of peptide strands of protein and decrease in hydrophobicity of gold nanoparticles. A ground state complex is also formed which is evident from the moderate shift observed in the absorbance peaks. Apparent association constant was also determined from the absorption spectra and was found to be maximum for HEWL and minimum for HSA. Gold nanoparticles were found to act as quenchers and reduced the protein fluorescence intensity. Binding constant and number of binding sites were found to be maximum for HEWL and minimum for HSA. The temperature dependent fluorescence studies were also performed to calculate the thermodynamic parameters and to determine the nature of interaction between the proteins and gold nanoparticles. The circular dichroism studies elucidate the reason behind the maximum binding for HEWL and minimum binding for HSA. TGA analysis determined the thermal stability of the samples. Fluorescence lifetime studies indicate static quenching of proteins. Antibacterial activity of protein-gold nanoparticles was studied against four pathogens, viz. Bacillus pumilus, Escherichia coli, Pseudomonas aeruginosa, and Staphylococcus aureus. HEWL exhibits a tunable antimicrobial activity against Pseudomonas aeruginosa due to the maximum binding of HEWL with gold nanoparticles. The study proposes a novel method for adjusting the antibacterial activity of HEWL against Pseudomonas aeruginosa when the resistance of this pathogen is a major issue in the chemotherapy of many infectious diseases. Thus the combination therapy of protein-gold nanoparticles could prove to be a new approach in medical field in the near future.

Antifungal and Cytotoxic Evaluation of Photochemically Synthesized Heparin-Coated Gold and Silver Nanoparticles

Heparin-based silver nanoparticles (AgHep-NPs) and gold nanoparticles (AuHep-NPs) were produced by a photochemical method using silver nitrate and chloroauric acid as metal precursors and UV light at 254 nm. UV-Vis spectroscopy graphs showed absorption for AgHep-NPs and AuHep-NPs at 420 nm and 530 nm, respectively. TEM revealed a pseudospherical morphology and a small size, corresponding to 10-25 nm for AgHep-NPs and 1.5-7.5 nm for AuHep-NPs. Their antifungal activity against Candida albicans, Issatchenkia orientalis (Candida krusei), and Candida parapsilosis was assessed by the microdilution method. We show that AgHep-NPs were effective in decreasing fungus density, whereas AuHep-NPs were not. Additionally, the viability of human gingival fibroblasts was preserved by both nanoparticle types at a level above 80%, indicating a slight cytotoxicity. These results are potentially useful for applications of the described NPs mainly in dentistry and, to a lesser extent, in other biomedical areas.

Glass/Au Composite Membranes with Gold Nanoparticles Synthesized inside Pores for Selective Ion Transport

Nanocomposite membranes have been actively developed in the last decade. The involvement of nanostructures can improve the permeability, selectivity, and anti-fouling properties of a membrane for improved filtration processes. In this work, we propose a novel type of ion-selective Glass/Au composite membrane based on porous glass (PG), which combines the advantages of porous media and promising selective properties. The latter are achieved by depositing gold nanoparticles into the membrane pores by the laser-induced liquid phase chemical deposition technique. Inside the pores, gold nanoparticles with an average diameter 25 nm were formed, which was confirmed by optical and microscopic studies. To study the transport and selective properties of the PG/Au composite membrane, the potentiometric method was applied. The uniform potential model was used to determine the surface charge from the experimental data. It was found that the formation of gold nanoparticles inside membrane pores leads to an increase in the surface charge from −2.75 mC/m² to −5.42 mC/m². The methods proposed in this work allow the creation of a whole family of composite materials based on porous glasses. In this case, conceptually, the synthesis of these materials will differ only in the selection of initial precursors.

Bio-application of Inorganic Nanomaterials in Tissue Engineering

Inorganic nanomaterials or nanoparticles (INPs) have drawn high attention for their usage in the biomedical field. In addition to the facile synthetic and modifiable property of INPs, INPs have various unique properties that originate from the components of the INPs, such as metal ions that are essential for the human body. Apart from their roles as components of the human body, inorganic materials have unique properties, such as magnetic, antibacterial, and piezoelectric, so that INPs have been widely used as either carriers or inducers. However, most of the bio-applicable INPs, especially those consisting of metal, can cause cytotoxicity. Therefore, INPs require modification to alleviate the harmful effect toward the cells by controlling the release of metal ions from INPs. Even though many attempts have been made to modify INPs, many things, including the side effects of INPs, still remain as obstacles in the bio-application, which need to be elucidated. In this chapter, we introduce novel INPs in terms of their synthetic method and bio-application in tissue engineering.

One-Step Photochemical Green Synthesis of Water-Dispersible Ag, Au, and Au@Ag Core-Shell Nanoparticles

A simple and green synthetic protocol for the rapid and effective preparation of Ag, Au and Au@Ag core-shell nanoparticles (NPs) is reported based on the light irradiation of a biocompatible, water-soluble dextran functionalized with benzophenone (BP) in the presence of AgNO₃ , HAuCl₄ , or both. Photoactivation of the BP moiety produces the highly reducing ketyl radicals through fast (<50 ns) intramolecular H-abstraction from the dextran scaffold, which, in turn, ensures excellent dispersibility of the obtained metal NPs in water. The antibacterial activity of the AgNPs and the photothermal action of the Au@Ag core-shell are also shown.

Light-Driven Shape Morphing, Assembly, and Motion of Nanocomposite Gel Surfers

Patterning of nanoparticles (NPs) via photochemical reduction within thermally responsive hydrogel films is demonstrated as a versatile platform for programming light-driven shape morphing and materials assembly. Responsive hydrogel disks, containing patterned metal NPs, form characteristic wrinkled structures when illuminated at an air/water interface. The resulting distortion of the three-phase (air/water/hydrogel) contact lines induces capillary interactions between two or more disks, which are either attractive or repulsive depending on the selected pattern of light. By programming the shapes of the NP-rich regions, as well as of the hydrogel objects themselves, the number and location of attractive interactions are specified, and the assembly geometry is controlled. Remarkably, appropriately patterned illumination enables sustained rotation and motion of the hydrogel disks. Overall, these results offer insight into a wide variety of shape-programmable materials and capillary assemblies, simply by controlling the NP patterns and illumination of these soft materials.

Conductive nanomaterials for 2D and 3D printed flexible electronics

This review describes recent developments in the field of conductive nanomaterials and their application in 2D and 3D printed flexible electronics, with particular emphasis on inks based on metal nanoparticles and nanowires, carbon nanotubes, and graphene sheets. We present the basic properties of these nanomaterials, their stabilization in dispersions, formulation of conductive inks and formation of conductive patterns on flexible substrates (polymers, paper, textile) by using various printing technologies and post-printing processes. Applications of conductive nanomaterials for fabrication of various 2D and 3D electronic devices are also briefly discussed.

Synthesis and characterization of size- and shape-controlled silver nanoparticles

Silver nanoparticles (AgNPs) have application potential in diverse areas ranging from wound healing to catalysis and sensing. The possibility for optimizing the physical, chemical and optical properties for an application by tailoring the shape and size of silver nanoparticles has motived much research on methods for synthesis of size- and shape-controlled AgNPs. The shape and size of AgNPs are reported to vary depending on choice of the Ag precursor salt, reducing agent, stabilizing agent and on the synthesis technique used. This chapter provides a detailed review on various synthesis approaches that may be used for synthesis of AgNPs of desired size and shape. Silver nanoparticles may be synthesized using diverse routes, including, physical, chemical, photochemical, biological and microwave -based techniques. Synthesis of AgNPs of diverse shapes, such as, nanospheres, nanorods, nanobars, nanoprisms, decahedral nanoparticles and triangular bipyramids is also discussed for chemical-, photochemical- and microwave-based synthesis routes. The choice of chemicals used for reduction and stabilization of nanoparticles is found to influence their shape and size significantly. A discussion on the mechanism of synthesis of AgNPs through nucleation and growth processes is discussed for AgNPs of varying shape and sizes so as to provide an insight on the various synthesis routes. Techniques, such as, electron microscopy, spectroscopy, and crystallography that can be used for characterizing the AgNPs formed in terms of their shape, sizes, crystal structure and chemical composition are also discussed in this chapter.

Graphical Abstract:

Quantum chemistry of OsCl62− photoaquation products and the reaction scheme

The mechanism of formation of the hydroxocomplex Os^IVCl₅(OH)²⁻ after photoexcitation of Os^IVCl⁶²⁻.

Kinetics and growth mechanism of the photoinduced synthesis of silver nanoparticles stabilized with lysozyme

A fast and single-step procedure is reported for the preparation of stable solutions of spherical-shaped silver nanoparticles (AgNPs) coated with lysozyme (LZ). The preparation of the AgNP@LZ nanocomposites was based on the reduction of Ag⁺ with ketyl radicals photo-generated by the UVA-photolysis of the benzoin I-2959. Both reaction precursors bind to LZ, modifying its superficial charge and conformational structure. The photo-induced kinetics of formation of the AgNPs as a function of the LZ concentration was monitored in-situ by UV-vis absorption spectroscopy. The multivariate curve resolution-alternating least square (MCR-ALS) method was used for the deconvolution of the kinetic curves for each transient species formed before the growth of the final AgNPs colloids. The Kolmogorov-Johnson-Mehl-Avrami (KJMA) model to describe the formation of the AgNPs was used, and the respective first-order rate constants for the growth of the AgNPs as a function of the lysozyme concentration were calculated and the role of the protein capping in the growth kinetics was evaluated. Despite the protein being partially oxidized by the photo-generated radicals, it was strongly adsorbed onto the silver surface forming a tight coating shell around the AgNPs of approximately 30-60 protein molecules. As a result of the partial denaturation and crowded packing, its intrinsic lytic activity was strongly reduced.

Bidirectional plasmonic coloration with gold nanoparticles by wavelength-switched photoredox reaction

Reversible tuning of localized plasmon resonance that is supported by nanometric-sized metal particles holds potentially huge benefits in terms of manipulating light for widespread photonic applications. Although the practice of altering the frequency and the amplitude of plasmon resonance on gold nanoparticles is relatively well established, dynamic tuning by all-optical approaches for coloration has long been hindered due to limited implementation approaches with which gold nanomaterials can be photosynthetically manipulated. Here, we develop a wavelength-switched photoredox approach and demonstrate bidirectional tuning of the plasmonic resonance of crystalline gold nanoparticles for reversible surface-plasmon-resonance-based coloration. The reversible plasmonic resonance control is achieved by a combination of photoreduction of gold ions and photooxidation of gold nanorods by switching the illumination between UV and near-UV-Vis light, respectively. As one example, the plasmon resonance peak of gold nanorods is reversibly tuned between 630 and 660 nm by switching the light wavelengths. Utilizing wavelength-switchable photoredox reactions, we demonstrate reversible color patterning by mask illuminating a gold nanorod sample solution. This approach offers not only an easy-to-implement method for realizing non-contact modulating plasmon-resonance based colors, but also new opportunities for reversibly tuning local plasmon resonance by all-optically shaping single nanoparticles. This holds great potential for a wide range of applications, including active-substrate-based surface-enhanced Raman scattering (SERS), erasable optical data storage and dynamic laser color printing, among others.

Understanding the CO Oxidation on Pt Nanoparticles Supported on MOFs by Operando XPS

Metal-organic frameworks (MOFs) are playing a key role in developing the next generation of heterogeneous catalysts. In this work, near ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) is applied to study in operando the CO oxidation on Pt@MOFs (UiO-67) and Pt@ZrO2 catalysts, revealing the same Pt surface dynamics under the stoichiometric CO/O2 ambient at 3 mbar. Upon the ignition at ca. 200 °C, the signature Pt binding energy (BE) shift towards the lower BE (from 71.8 to 71.2 eV) is observed for all catalysts, confirming metallic Pt nanoparticles (NPs) as the active phase. Additionally, the plug-flow light-off experiments show the superior activity of the Pt@MOFs catalyst in CO oxidation than the control Pt@ZrO2 catalyst with ca. 28 % drop in the T 50% light-off temperature, as well as high stability, due to their sintering-resistance feature. These results provide evidence that the uniqueness of MOFs as the catalyst supports lies in the structural confinement effect.

Rapid in situ synthesis of polymer-metal nanocomposite films in several seconds using a CO2 laser

We demonstrate the rapid in situ synthesis of polymer-metal nanocomposite films using a CO2 laser at 10.6 μm. The mechanism of our method is that the precursor of the metal nanoparticles, i.e., the metallic ions, is very rapidly reduced in the laser-heated polymer matrix without any reducing agent. Unlike other known laser-induced reduction methods using UV lasers, which produce radicals to promote reduction, the CO2 laser energy is mainly absorbed by the glass substrate, and the laser-heated substrate heats the polymer matrix through heat diffusion to promote reduction. The superiority of the use of CO2 lasers over nanosecond visible~UV lasers is also demonstrated in terms of the damage to the film. The developed method can be a new alternative to quickly synthesize a variety of polymer-metal nanocomposite films.

Direct sunlight enabled photo-biochemical synthesis of silver nanoparticles and their Bactericidal Efficacy: Photon energy as key for size and distribution control

It is highly desirable to discover novel green synthesis methods for cheap and scalable synthesis of nanoparticles (NPs) to reduce the negative impact on the environment. But these approaches generally impose great challenge in controlling size, shape, and homogeneity of product NPs. Here in the present study, we report a novel approach enabling direct sunlight and oyster mushroom (Pleurotus citrinopileatus) extract for the photo-biochemical synthesis of Ag NPs. Sunlight of different wavelength was used to control the size and distribution of photo-biochemically produced NPs. Interestingly, it is observed that a smaller wavelength of sunlight produces smaller sized of NPs with a narrow size distribution. For examples; blue sunlight produces colloidal silver NPs with an average diameter of ~ 3.28 nm and 0.72 nm size distribution, while full sunlight produces comparatively larger sized (7.08 nm) NPs with wider (2.92 nm) size distribution. Since present approach uses only direct sunlight, freely available renewable energy source, a cheap biological extract as reducing and capping agent and cheap silver precursor, therefore it is an environment-friendly approach and can be used for the synthesis of NPs at industrial scale. Moreover, the size-dependent bactericidal effect has also been studied against pathogenic, Escherichia coli, bacteria. The minimum inhibitory concentration (MIC) 25 ppm and MBC 30 ppm have been observed for silver NPs of 3.28 nm average diameter.

Colorimetric Detection of Mercury(II) Ion in Aqueous Solution Using Silver Nanoparticles

Novel green-chemistry synthesis of silver nanoparticles (AgNPs) is introduced as a low-cost, rapid and easy-to-use analytical method for mercury ion detection. Aqueous fruit extract of water apple (Syzygium aqueum) was used for the first time as bioreductant to synthesize stable AgNPs. The prepared AgNPs have a yellowish-brown color with a surface plasmon resonance peak at 420 nm. The addition of Hg(II) ions then changes the AgNPs color to colorless. The color change was in proportion to the concentration of Hg(II) ions. The presence of other metal ions in the system was also evaluated. The proposed method shows good selectivity and sensitivity towards Hg(II) ions. Using UV-visible spectrophotometry, the detection limit of the developed method was 8.5 × 10^-7 M. The proposed method has been successfully applied for determination of Hg(II) ions in tap and lake water samples with precision better than 5%.

Development of Ag/AgX (X = Cl, I) nanoparticles toward antimicrobial, UV-protected and self-cleanable viscose fibers

In situ synthesis of Ag/AgX nanoparticles (NPs) onto viscose fibers adds new functionalities and broadens their applications. In this study, Ag/AgX (X = Cl, I) NPs were in situ synthesized onto viscose fibers to impart brilliant colors, UV-protection, antimicrobial, self-cleaning, and photocatalytic properties. The AgX NPs were deposited on the fibers by ultrasonic irradiation, while Ag-NPs were formed by photoreduction of excess Ag⁺ ions under UV irradiation. The Ag/AgX NPs-loaded onto viscose fibers endowed with pale yellow for Ag/AgI and pale purple/violet for Ag/AgCl. The colored viscose fibers showed excellent antimicrobial activity against Escherichia coli (gram-negative), Staphylococcus aureus (Gram positive), and Candida Albican. The Ag/AgX/viscose fiber also showed excellent photocatalytic and self-cleaning activity toward degradation of methylene blue.

Facile synthesis of dispersed Ag nanoparticles on chitosan-TiO2 composites as recyclable nanocatalysts for 4-nitrophenol reduction

This paper presents a facile, rapid, and controllable procedure for the recovery of trace Ag⁺ ions and in situ assembly of well dispersed Ag nanoparticles on chitosan-TiO₂ composites through bioaffinity adsorption followed by photocatalytic reduction. The prepared Ag nanoparticles are proven to be efficient and recyclable nanocatalysts for the reduction of 4-nitrophenol to 4-aminophenol in the presence of NaBH₄. Well dispersed quasi-spherical Ag NPs are synthesized in 20 min in the designed inner-irradiated photocatalytic system under a wide range of Ag⁺ concentrations (50-200 mg l^-1), temperatures (10 °C-25 °C) conditions, and UV or visible light irradiation. The synthesized Ag NPs can catalyze the reduction of 4-nitrophenol by NaBH₄ at 100% conversion in 120 min and preserve the catalytic activity in five successive cycles. This procedure for trace Ag⁺ ions recovery and Ag NPs assembly has the potential to be scaled up for the mass production of recyclable Ag nanocatalysts. The present work provides a green and efficient procedure for the conversion of hazardous 4-nitrophenol to industrially important 4-aminophenol and also sheds a light on designing scaled-up procedures for treating high volumes of wastewater with dilute heavy metals to produce recyclable metallic nanocatalysts in aqueous systems.

Benzil/triethylamine: a photo-reducing system for Cu2

Abstract

We have investigated the photo-induced reduction of Cu²⁺–Cu⁰ using benzil/triethylamine mixtures. The formation of elemental Cu is indicated by the appearance of its characteristic plasmon absorption peaks at 515 nm and 620 nm. Importantly, the nature of the counterion of the Cu²⁺ salt affects the reduction process. In the presence of Cl⁻, the reduction proceeds faster than with SO₄^2-. Photo-induced electron transfer between excited benzil and triethylamine leads to the benzil radical anion, which acts as the reducing agent for Cu²⁺ and generates Cu⁰.

Graphical abstract

Electronic supplementary material

The online version of this article (10.1007/s00706-017-2085-7) contains supplementary material, which is available to authorized users.

A signal-on, colorimetric determination of deoxyribonuclease I activity utilizing the photoinduced synthesis of gold nanoparticles

A simple, colorimetric method is developed for the determination of deoxyribonuclease I (DNase I) activity based on the novel finding that DNase I can promote the photoinduced synthesis of gold nanoparticles (AuNPs). In the absence of DNase I, a phosphorothioate (PS) DNA probe remains intact and captures Au(iii) through a strong Au-thiol interaction, which prevents the photoinduced synthesis of AuNPs, leaving the sample in a colorless state. On the other hand, in the presence of DNase I, the PS DNA probe is cleaved into small fragments that are removed via a simple purification process. The resulting solution, after the incubation with HAuCl₄ and threonine (Thr), forms AuNPs by UV light irradiation with the aid of Thr which acts as a catalyst for the Au(iii) reduction process. As a result, a red-colored suspension is produced. By monitoring the color changes of the samples with the naked eye, the DNase I activity was conveniently determined. In addition, the clinical utility of this simple, yet highly efficient colorimetric strategy was verified by reliably quantifying the DNase I activities in a bovine urine sample. Importantly, the working principle designed for the determination of DNase I activity was successfully expanded for the detection of target nucleic acids, ensuring the universal applicability of the developed assay system.

Dense ionization and subsequent non-homogeneous radical-mediated chemistry of femtosecond laser-induced low density plasma in aqueous solutions: synthesis of colloidal gold

The "cold" low density plasma channels generated by the filamentation of powerful femtosecond laser pulses in aqueous solutions constitute a source of dense ionization. Here, we probed the radiation-assisted chemistry of water triggered by laser ionization via the radical-mediated synthesis of nanoparticles in gold chloride aqueous solutions. We showed that the formation of colloidal gold originates from the reduction of trivalent ionic gold initially present in solution by the reactive radicals (e.g. hydrated electrons) produced upon the photolysis of water. We analyzed both the reaction kinetics of the laser-induced hydrated electrons and the growth kinetics of the gold nanoparticles. Introduction of radical scavengers into the solutions and different initial concentrations of gold chloride provided information about the radical-mediated chemistry. The dense ionization results in the second order cross-recombination of the photolysis primary byproducts. Competition with recombination imposes the non-homogeneous interaction of reactive radicals with solute present in irradiated aqueous solutions. Such a laser-induced non-homogeneous chemistry suggests similarities with the radiation chemistry of water exposed to conventional densely ionizing radiation (high dose rate, high linear energy transfer).

Additive manufacture of complex 3D Au-containing nanocomposites by simultaneous two-photon polymerisation and photoreduction

The fabrication of complex three-dimensional gold-containing nanocomposite structures by simultaneous two-photon polymerisation and photoreduction is demonstrated. Increased salt delivers reduced feature sizes down to line widths as small as 78 nm, a level of structural intricacy that represents a significant advance in fabrication complexity. The development of a general methodology to efficiently mix pentaerythritol triacrylate (PETA) with gold chloride hydrate (HAuCl4∙3H2O) is reported, where the gold salt concentration is adjustable on demand from zero to 20 wt%. For the first-time 7-Diethylamino-3-thenoylcoumarin (DETC) is used as the photoinitiator. Only 0.5 wt% of DETC was required to promote both polymerisation and photoreduction of up to 20 wt% of gold salt. This efficiency is the highest reported for Au-containing composite fabrication by two-photon lithography. Transmission Electron Microscopy (TEM) analysis confirmed the presence of small metallic nanoparticles (5.4 ± 1.4 nm for long axis / 3.7 ± 0.9 nm for short axis) embedded within the polymer matrix, whilst X-ray Photoelectron Spectroscopy (XPS) confirmed that they exist in the zero valent oxidation state. UV-vis spectroscopy defined that they exhibit the property of localised surface plasmon resonance (LSPR). The capability demonstrated in this study opens up new avenues for a range of applications, including plasmonics, metamaterials, flexible electronics and biosensors.

Direct Metal Writing and Precise Positioning of Gold Nanoparticles within Microfluidic Channels for SERS Sensing of Gaseous Analytes

We demonstrate a one-step precise direct metal writing of well-defined and densely packed gold nanoparticle (AuNP) patterns with tunable physical and optical properties. We achieve this by using two-photon lithography on a Au precursor comprising poly(vinylpyrrolidone) (PVP) and ethylene glycol (EG), where EG promotes higher reduction rates of Au(III) salt via polyol reduction. Hence, clusters of monodisperse AuNP are generated along raster scanning of the laser, forming high-particle-density, well-defined structures. By varying the PVP concentration, we tune the AuNP size from 27.3 to 65.0 nm and the density from 172 to 965 particles/μm², corresponding to a surface roughness of 12.9 to 67.1 nm, which is important for surface-based applications such as surface-enhanced Raman scattering (SERS). We find that the microstructures exhibit an SERS enhancement factor of >10⁵ and demonstrate remote writing of well-defined Au microstructures within a microfluidic channel for the SERS detection of gaseous molecules. We showcase in situ SERS monitoring of gaseous 4-methylbenzenethiol and real-time detection of multiple small gaseous species with no specific affinity to Au. This one-step, laser-induced fabrication of AuNP microstructures ignites a plethora of possibilities to position desired patterns directly onto or within most surfaces for the future creation of multifunctional lab-on-a-chip devices.

Generating highly reflective and conductive metal layers through a light-assisted synthesis and assembling of silver nanoparticles in a polymer matrix

The development of metalized surfaces exhibiting mirror properties and/or electric conductivity without heavy equipments and with low metal charge is a big challenge in view of many industrial applications. We report herein on the photo-assembling of silver nanoparticles (AgNPs) in a polymer matrix, carried out within minutes from an acrylate monomer and silver nitrate at room temperature, under air and without any solvents. The top surface of the material gets converted into a continuous silver thin film and a depthwise concentration gradient of AgNPs is created in the polymer, which images the absorption profile of the actinic UV light in the reactive formulation. This specific assembling of the silver@polymer coating induces excellent reflective and conductive properties. The conductance was observed to strongly increase with increasing the exposure from 3 to 30 min due to the formation of a more and more compact metal film. This coating strategy works with a variety of substrates (textile, paper, glass, wood, plastic and stainless steel). Moreover, on flexible surfaces such as textile, the flexibility was preserved. The possibility to use this kind of nanomaterial as a printing ink, with a much lower metal concentration (3 to 5 wt.%) than concurrent inks, was also demonstrated.

One-Pot Hybrid SnO2 /Poly(methyl methacrylate) Nanocomposite Formation through Pulsed Laser Irradiation

The localized in situ formation of tin dioxide (SnO₂ ) nanoparticles embedded in poly(methyl methacrylate) (PMMA) films is presented. This is achieved by the photoinduced conversion of the tin acetate precursor included in polymeric films, through controlled UV or visible pulsed laser irradiation at λ=355 and 532 nm, respectively. The evolution of the formation of nanoparticles is followed by UV/Vis spectroscopy and shows that their growth is affected in different ways by the laser pulses at the two applied wavelengths. This, in combination with electron microscopy analysis, reveals that, depending on the irradiation wavelength, the size of the nanoparticles in the final nanocomposites differs. This difference is attributed to distinct mechanistic pathways that lead to the synthesis of small nanoparticles (from 1.5 to 4.5 nm) at λ=355 nm, whereas bigger ones (from 5 to 16 nm) are formed at λ=532 nm. At the same time, structural studies with both X-ray and electron diffraction measurements demonstrate the crystallinity of SnO₂ nanoparticles in both cases, whereas XPS analysis confirms the light-induced oxidation of tin acetate into SnO₂ . Taken all together, it is demonstrated that the pulsed laser irradiation at λ=355 and 532 nm leads to the formation of SnO₂ nanoparticles with defined features highly dispersed in PMMA solid matrices.

UV-Curable Aliphatic Silicone Acrylate Organic-Inorganic Hybrid Coatings with Antibacterial Activity

The most effective means to protect against bacterial invasion and to reduce the risk of healthcare-associated infections are antibacterial components synthesis. In this study, a novel process for the synthesis of organic–inorganic hybrid coatings containing silver nanoparticles is presented. Silver nanoparticles and polymer formation proceeds simultaneously through the in situ photoreduction of silver salt to silver nanoparticles and UV-crosslinking of bifunctional aliphatic silicone acrylate. The nanocomposite films with 0.5–1.43 wt % of silver nanoparticles concentration were obtained and investigated. The formation of silver nanoparticles in polymer matrix was confirmed via UV-visible spectroscopy, X-ray diffraction (XRD), Fourier transform infrared spectroscopy, scanning electron spectroscopy, and energy dispersive spectroscopy. Our investigations clearly show the formation of silver nanoparticles in silicone acrylate network. Direct photoreduction of silver salt by UV-radiation in the organic media produced silver nanoparticles exhibiting cubic crystal structure. The size of nanoparticles was determined to be near 20 ± 5 nm. The antibacterial activities of coatings were determined using the disc diffusion and direct contact methods. UV-curable silicone acrylate hybrid coatings exhibited antibacterial activity against harmful bacteria strains.

Photochemical synthesis of silver nanoparticles on chitosans/montmorillonite nanocomposite films and antibacterial activity

Silver nanoparticles (AgNPs) were synthetized on chitosans/montmorillonite nanocomposite films by photochemical method. Nanocomposites were prepared using chitosans with different molar masses and deacetylation degrees, as well as modified with diethylaminoethyl (DEAE) and dodecyl groups. AgNPs formation on the films was followed by the appearance of the plasmon band around 440nm as a function of irradiation time. TEM images revealed AgNPs with spherical morphology for all nanocomposites. For nanocomposites using modified chitosans, the AgNPs synthesis occurred quickly (1.5h) while for the others films it was above 11h. The film of modified chitosan with dodecyl and DEAE groups presented smaller and more uniform nanoparticles size along mixture of exfoliated and intercalated structures. This modified chitosan is an amphiphilic compound that can act controlling the size/shape of the AgNPs. The results of antibacterial activity suggested that all nanocomposite-AgNPs films inhibited the growth of Escherichia coli and Bacillus subtilis.