The role of the light source in antimicrobial photodynamic therapy.

Antimicrobial photodynamic therapy (APDT) is a promising approach to fight the growing problem of antimicrobial resistance that threatens health care, food security and agriculture. APDT uses light to excite a light-activated chemical (photosensitiser), leading to the generation of reactive oxygen species (ROS). Many APDT studies confirm its efficacy in vitro and in vivo against bacteria, fungi, viruses and parasites. However, the development of the field is focused on exploring potential targets and developing new photosensitisers. The role of light, a crucial element for ROS production, has been neglected. What are the main parameters essential for effective photosensitiser activation? Does an optimal light radiant exposure exist? And finally, which light source is best? Many reports have described the promising antibacterial effects of APDT in vitro, however, its application in vivo, especially in clinical settings remains very limited. The restricted availability may partially be due to a lack of standard conditions or protocols, arising from the diversity of selected photosensitising agents (PS), variable testing conditions including light sources used for PS activation and methods of measuring anti-bacterial activity and their effectiveness in treating bacterial infections. We thus sought to systematically review and examine the evidence from existing studies on APDT associated with the light source used. We show how the reduction of pathogens depends on the light source applied, radiant exposure and irradiance of light used, and type of pathogen, and so critically appraise the current state of development of APDT and areas to be addressed in future studies. We anticipate that further standardisation of the experimental conditions will help the field advance, and suggest key optical and biological parameters that should be reported in all APDT studies. More in vivo and clinical studies are needed and are expected to be facilitated by advances in light sources, leading to APDT becoming a sustainable, alternative therapeutic option for bacterial and other microbial infections in the future.

Hyper Rayleigh scattering from DNA nucleotides in aqueous solution.

Nucleotides are organic compounds consisting of a phosphate group, a nitrogenous base, namely adenine (A), thymine (T), cytosine (C), or guanine (G), and a sugar, here deoxyribose. The magnitude of the first hyperpolarizability ß of these four DNA nucleotides was determined in aqueous solution with the nonlinear optical technique of hyper rayleigh scattering under non resonant conditions at a fundamental wavelength of 800 nm. The smallest value is found to be 1.67 ± 0.15 × 10-30 esu for thymidine-5'-monophosphate and the highest is 1.76 ± 0.16 × 10-30 esu for 2'-guanosine-5'-monophosphate. Polarization resolved studies were also performed to question the symmetry of the first hyperpolarizability tensor and access the ratio of some elements of the first hyperpolarizability tensor. These experimental results were then compared to the theoretical values of these first hyperpolarizabilities obtained with the density functional theory at the level of the PCM-B3LYP/6-31G+(d) basis and taking into account the solvent.

Nonlinear Optical Absorption in Nanoscale Films Revealed through Ultrafast Acoustics.

Herein we describe a novel spinning pump-probe photoacoustic technique developed to study nonlinear absorption in thin films. As a test case, an organic polycrystalline thin film of quinacridone, a well-known pigment, with a thickness in the tens of nanometers range, is excited by a femtosecond laser pulse which generates a time-domain Brillouin scattering signal. This signal is directly related to the strain wave launched from the film into the substrate and can be used to quantitatively extract the nonlinear optical absorption properties of the film itself. Quinacridone exhibits both quadratic and cubic laser fluence dependence regimes which we show to correspond to two- and three-photon absorption processes. This technique can be broadly applied to materials that are difficult or impossible to characterize with conventional transmittance-based measurements including materials at the nanoscale, prone to laser damage, with very weak nonlinear properties, opaque, or highly scattering.

Two-photon absorption properties of multipolar triarylamino/tosylamido 1,1,4,4-tetracyanobutadienes.

The synthesis and characterization of four new tetracyanobutadiene (TCBD) derivatives (1, 3c and 4b-c) incorporating tosylamido and 4-triphenylamino moieties are reported. Along with those of five closely related or differently branched TCBDs derivatives (2, 3a-b, 4c and 5), their linear and (third-order) nonlinear optical properties were investigated by electronic absorption spectroscopy and Z-scan measurements. Among these compounds, the tri-branched compounds 3c and 5 are the most active two-photon absorbers, with effective cross-sections of 275 and 350 GM at 900 nm, respectively. These properties are briefly discussed with the help of DFT calculations, focussing on structural and electronic factors, and contextualized with results obtained previously for related compounds.

Prospects for More Efficient Multi-Photon Absorption Photosensitizers Exhibiting Both Reactive Oxygen Species Generation and Luminescence.

The use of two-photon absorption (TPA) for such applications as microscopy, imaging, and photodynamic therapy (PDT) offers several advantages over the usual one-photon excitation. This creates a need for photosensitizers that exhibit both strong two-photon absorption and the highly efficient generation of reactive oxygen species (ROS), as well as, ideally, bright luminescence. This review focuses on different strategies utilized to improve the TPA properties of various multi-photon absorbing species that have the required photophysical properties. Along with well-known families of photosensitizers, including porphyrins, we also describe other promising organic and organometallic structures and more complex systems involving organic and inorganic nanoparticles. We concentrate on the published studies that provide two-photon absorption cross-section values and the singlet oxygen (or other ROS) and luminescence quantum yields, which are crucial for potential use within PDT and diagnostics. We hope that this review will aid in the design and modification of novel TPA photosensitizers, which can help in exploiting the features of nonlinear absorption processes.

Biogenic Gold Nanoparticles Decrease Methylene Blue Photobleaching and Enhance Antimicrobial Photodynamic Therapy.

Antibiotic resistance is a growing concern that is driving the exploration of alternative ways of killing bacteria. Here we show that gold nanoparticles synthesized by the mycelium of Mucor plumbeus are an effective medium for antimicrobial photodynamic therapy (PDT). These particles are spherical in shape, uniformly distributed without any significant agglomeration, and show a single plasmon band at 522-523 nm. The nanoparticle sizes range from 13 to 25 nm, and possess an average size of 17 ± 4 nm. In PDT, light (from a source consisting of nine LEDs with a peak wavelength of 640 nm and FWMH 20 nm arranged in a 3 × 3 array), a photosensitiser (methylene blue), and oxygen are used to kill undesired cells. We show that the biogenic nanoparticles enhance the effectiveness of the photosensitiser, methylene blue, and so can be used to kill both Gram-positive (Staphylococcus aureus) and Gram-negative (Escherichia coli) bacteria. The enhanced effectiveness means that we could kill these bacteria with a simple, small LED-based light source. We show that the biogenic gold nanoparticles prevent fast photobleaching, thereby enhancing the photoactivity of the methylene blue (MB) molecules and their bactericidal effect.

Reversible Photocontrol of DNA Melting by Visible-Light-Responsive F4-Coordinated Azobenzene Compounds.

Spatiotemporal control over the regulation of intra- and intermolecular motions in naturally occurring systems is systematically studied to expand the toolbox of mechanical operations in multicomponent nanoarchitectures. DNA is ideally suited for programming light-powered processes that are based on a minimalist molecular design. Here, the noncovalent incorporation of bistable photoswitches into B-like DNA moieties is shown to trigger the thermal transition midpoint of the duplexes by converting visible light into directed mechanical work by orchestrating the collective actions of the photoresponsive chromophores and the host DNA nanostructures. Besides its practical applications, the resulting hybrid nanosystem bears unique features of modulability, biocompatibility, reversibility, and addressability, which are key components for developing molecular photon-controlled programmed materials.

Two-Photon Imaging of 3D Organization of Bimetallic AuAg Nanoclusters in DNA Matrix.

We report on two-photon excitation properties of small silver-doped gold nanoclusters (AuAgNCs) and on their three-dimensional arrangement in a hybrid system composed of DNA liquid crystals (LCs) and AuAgNCs. UV-vis and fluorescence spectroscopy, transmission electron microscopy (TEM), and multiphoton excitation spectroscopy were used to characterize the nanoparticles. We show that AuAgNCs exhibit two-photon excited luminescence (2PL) emission and second-harmonic generation (SHG) and that these properties remain the same in liquid crystalline matrix. The results are described in detail and discussed in the context of possible imaging application of AuAgNC and specific AuAgNCs organization induced by liquid crystalline ordering of DNA molecules.

Linear Optical and Third-Order Nonlinear Optical Properties of Some Fluorenyl- and Triarylamine-Containing Tetracyanobutadiene Derivatives.

The synthesis and characterization of four new tetracyanobutadiene (TCBD) derivatives (1-3 and 2') incorporating 2- or 2,7-fluorenyl and diphenylamino moieties are reported. The electroactivity of 1-3 and 2' was studied by cyclic voltammetry (CV), while the linear optical and (third-order) nonlinear optical (NLO) properties were investigated by electronic spectroscopy and Z-scan studies, respectively. All experimental investigations were rationalized by DFT computations, providing an insight into the electronic structure of these derivatives and on their application potential. We show that these derivatives are nonluminescent in solution at ambient temperatures, but become fluorescent in solvent glasses. This finding constitutes an unprecedented observation for TCBD derivatives. Also, we show by Z-scan studies that these derivatives behave as two-photon absorbers in the near-IR range (800-1050 nm). These third-order NLO properties are discussed and compared with those of their alkynyl precursors (4-6), which have been investigated by two-photon excited fluorescence (TPEF).

Unravelling the Binding Mechanism of a Poly(cationic) Anthracenyl Fluorescent Probe with High Affinity toward Double-Stranded DNA.

We report the synthesis, spectroscopy, and the DNA binding properties of a biocompatible, water-soluble, polycationic two-photon absorbing anthracenyl derivative (Ant-PIm) specifically designed for biorelated applications. Detailed insights into the Ant-PIm-DNA binding interaction are provided by using several spectroscopic approaches, including UV-vis absorption, circular dichroism (CD), Fourier-transform infrared spectroscopy (FTIR), steady-state, and time-resolved fluorescence techniques. Absorption and fluorescence quantitative data analysis show a strong Ant-PIm-duplex interaction with binding constants of Kf = 4.7 ± 0.2 × 105 M-1, 7.1 ± 0.3 × 105 M-1, and 1.0 ± 0.1 × 106 M-1 at 298, 304, and 310 K, respectively. Spectral changes observed upon DNA binding provide evidence for a complex formation with off-on fluorescence pattern, which can be related to two consecutive binding equilibria. Results of DNA binders displacement and iodide quenching experimental assays unambiguously point to the groove binding mode of Ant-PIm to the DNA-helicate. Thermodynamic and chemical denaturation studies suggest that long-range interactions of hydrophobic nature regulate the association of Ant-PIm with the biopolymer. The ionic strength dependence of the binding constant shows that electrostatic component has an important contribution to the overall Gibbs free energy. FTIR and CD data provide evidence of partial modification of the B-DNA secondary structure, while the increase in the melting temperature clearly indicates the enhancement of the thermal stability of the duplex. Furthermore, the two-photon absorption cross section spectrum determined using the two-photon excited fluorescence (TPEF) technique shows a strong 2PA maximum at 820 nm with a σ2 > 800 GM, which emphasizes the advantageous combination of biological and optical properties possessed by this positively charged bioprobe.

Stabilization of DNA liquid crystals on doping with gold nanorods.

We report on the impact of doping with gold nanorods (NRs) on the formation and stability of DNA liquid crystals (LCs). Cetyl trimethylammonium (CTAB)-stabilized gold NRs were synthesized using the wet chemistry method. Different textures of cholesteric and columnar mesophases, as well as phase transitions, were observed using a polarized light microscope. It was found that liquid crystalline phases formed in the samples were qualitatively the same and the phase appearance sequence was preserved in the samples regardless of the doping. We show that depending on the concentration of gold NRs present in the phase, nanoparticle-doped cholesteric and columnar hexagonal phases existed in wider temperature ranges compared to pure DNA LCs. The potential applications of these liquid crystal-nanoparticle hybrid systems may include the fabrication of new optoelectronic devices and sensors.

Bio-mediated synthesis, characterization and cytotoxicity of gold nanoparticles.

We report here a "green" approach for the synthesis of gold nanoparticles (GNPs) in which the Mentha piperita extract was applied for the bioreduction of chloroauric acid and the stabilization of the formed nanostructures. The obtained GNPs were characterized by UV-Vis absorption spectroscopy and transmission electron microscopy (TEM). The reduction of gold ions with the plant extract leads to the production of nanoparticles with various shapes (spherical, triangular and hexagonal) and sizes (from 10 to 300 nm). The kinetics of the reaction was monitored and various conditions of the synthesis were investigated. As a result, we established protocols optimized towards the synthesis of nanospheres and nanoprisms of gold. The cytotoxic effect of the obtained gold nanoparticles was studied by performing MTT assay, which showed lower cytotoxicity of the biosynthesized GNPs compared to gold nanorods synthesized using the usual seed-mediated growth. The results suggest that the synthesis using plant extracts may be a useful method to produce gold nanostructures for various biological and medical applications.

Interactions of a biocompatible water-soluble anthracenyl polymer derivative with double-stranded DNA.

We have studied the interaction of a polymeric water soluble anthracenyl derivative () with salmon testes DNA. The results from UV-Vis, fluorescence, Fourier transform infrared (FT-IR) and circular dichroism spectroscopies indicate that the groove binding process regulates the interaction between and DNA. The binding constants, calculated by absorption spectroscopy at 298, 304 and 310 K, were equal to 3.2 × 10(5) M(-1), 4.7 × 10(5) M(-1), and 6.6 × 10(5) M(-1) respectively, proving a relatively high affinity of for salmon testes DNA. Results of Hoechst 33258 displacement assays strongly support the groove binding mode of to DNA. The association stoichiometry of the :DNA adduct was found to be 1 for every 5 base pairs. FT-IR spectra, recorded at different /DNA molar ratios, indicate the involvement of the phosphate groups and adenine and thymine DNA bases in the association process. Thermodynamic results suggest that hydrophobic forces regulate the binding of with DNA without excluding some extent of involvement of van der Waals forces and hydrogen bonding arising due to surface binding between the hydrophilic polymeric arms of the ligand and the functional groups positioned on the edge of the groove. The resulting composite biomaterial could constitute a valuable candidate for future biological and/or photonic applications.

Post-synthesis reshaping of gold nanorods using a femtosecond laser.

This work describes the interaction between femtosecond laser pulses (~130 fs, 800 nm) and gold nanorods (NRs) leading to reshaping of the NRs. We focus on the investigation of structural changes of the NRs and the parameters influencing the reshaping, like surface modification using sodium sulphide, laser power and the position of the longitudinal surface plasmon resonance band (l-SPR) with respect to the laser wavelength. A thermogravimetric analysis experiment is performed to examine changes in the composition of NRs upon heating. A new type of banana-shaped NPs is described and the conditions of their appearance are discussed.

A 5-(difluorenyl)-1,10-phenanthroline-based Ru(II) complex as a coating agent for potential multifunctional gold nanoparticles.

The synthesis and photophysical properties of small gold nanoparticles (NPs, AuNP-[Ru-PFF]) surface functionalized by 5-substituted-1,10-phenanthroline-ligand based Ru(II) complexes are described. Luminescence of the grafted and confined Ru(II) complexes is totally quenched on the gold surface. Nonlinear optical properties were determined via Z-scan measurements in the range 600-1300 nm for both the free Ru(II) complex and the related NPs. In the short wavelength range (around 600 nm) the behaviour switches from that of two-photon absorption (2PA) for the complex to saturable absorption for the NPs. 2PA applications such as optical power limiting or two-photon dioxygen sensitization can be anticipated for these nanoplatforms.

Three-Dimensional Imaging of Bioinspired Lipidic Mesophases Using Multicolored Light-Emitting Carbon Nanodots.

Recent progress in the design of carbon nanostructures exhibiting strong multiphoton-excited emission opens new pathways to explore the self-organization of lipids found in living organisms. Phospholipid-based lyotropic myelin figures (MFs) are promising materials as simplified models of biomembranes due to their structural resemblance to a multilamellar sheath insulating the axon. This study demonstrates the possibility of selective labeling of MFs by strongly emitting multicolor phloroglucinol-derived carbon nanodots (PG CNDs). Such dopants are efficiently excited by visible and near-infrared light; therefore, one- and two-photon fluorescence microscopies are incorporated to gain 3D insights into the MFs. Combining nondestructive fluorescence microscopy and spectroscopy techniques along with polarized light microscopy gives details on the stability and morphology of lipidic mesophases. Our findings suggest that PG CNDs can be a viable and simple alternative to conventional fluorescent lipid stains to image biologically relevant phospholipid-based structures.

Advancements in molecular disassembly of optical probes: a paradigm shift in sensing, bioimaging, and therapeutics.

The majority of self-assembled fluorescent dyes suffer from aggregation-caused quenching (ACQ), which detrimentally affects their diagnostic and therapeutic effectiveness. While aggregation-induced emission (AIE) active dyes offer a promising solution to overcome this limitation, they may face significant challenges as the intracellular environment often prevents aggregation, leading to disassembly and posing challenges for AIE fluorogens. Recent progress in signal amplification through the disassembly of ACQ dyes has opened new avenues for creating ultrasensitive optical sensors and enhancing phototherapeutic outcomes. These advances are well-aligned with cutting-edge technologies such as single-molecule microscopy and targeted molecular therapies. This work explores the concept of disaggregation-induced emission (DIE), showcasing the revolutionary capabilities of DIE-based dyes from their design to their application in sensing, bioimaging, disease monitoring, and treatment in both cellular and animal models. Our objective is to provide an in-depth comparison of aggregation versus disaggregation mechanisms, aiming to stimulate further advancements in the design and utilization of ACQ fluorescent dyes through DIE technology. This initiative is poised to catalyze scientific progress across a broad spectrum of disciplines.

Impact of the synergistic collaboration of oligothiophene bridges and ruthenium complexes on the optical properties of dumbbell-shaped compounds.

The linear and non-linear optical properties of a family of dumbbell-shaped dinuclear complexes, in which an oligothiophene chain with various numbers of rings (1, 3, and 6) acts as a bridge between two homoleptic tris(2,2'-bipyridine)ruthenium(II) complexes, have been fully investigated by using a range of spectroscopic techniques (absorption and luminescence, transient absorption, Raman, and non-linear absorption), together with density functional theory calculations. Our results shed light on the impact of the synergistic collaboration between the electronic structures of the two chemical moieties on the optical properties of these materials. Experiments on the linear optical properties of these compounds indicated that the length of the oligothiophene bridge was critical for luminescent behavior. Indeed, no emission was detected for compounds with long oligothiophene bridges (compounds 3 and 4, with 3 and 6 thiophene rings, respectively), owing to the presence of the (3)π-π* state of the conjugated bridge below the (3)MLCT-emitting states of the end-capping Ru(II) complexes. In contrast, the compound with the shortest bridge (2, one thiophene ring) shows excellent photophysical features. Non-linear optical experiments showed that the investigated compounds were strong non-linear absorbers in wide energy ranges. Indeed, their non-linear absorption was augmented upon increasing the length of the oligothiophene bridge. In particular, the compound with the longest oligothiophene bridge not only showed strong two-photon absorption (TPA) but also noteworthy three-photon-absorption behavior, with a cross-section value of 4×10(-78) cm(6) s(2) at 1450 nm. This characteristic was complemented by the strong excited-state absorption (ESA) that was observed for compounds 3 and 4. As a matter of fact, the overlap between the non-linear absorption and ESA establishes compounds 3 and 4 as good candidates for optical-power-limiting applications.

Remarkable effect of iridium cyclometalation on the nonlinear absorption properties of a quadrupolar imine ligand.

A new type of dinuclear iridium complex, based on a quadrupolar Schiff base ligand, is synthesized and its structure fully characterized. Its linear and nonlinear spectroscopic properties are investigated, evidencing a strong contribution of the metal-to-ligand transitions not only to the linear absorption but also to the two- and three-photon absorption properties.

Treatment of antibiotic-resistant bacteria colonizing diabetic foot ulcers by OLED induced antimicrobial photodynamic therapy.

We evaluate the efficacy of antimicrobial Photodynamic Therapy (APDT) for inactivating a variety of antibiotic-resistant clinical strains from diabetic foot ulcers. Here we are focused on APDT based on organic light-emitting diodes (OLED). The wound swabs from ten patients diagnosed with diabetic foot ulcers were collected and 32 clinical strains comprising 22 bacterial species were obtained. The isolated strains were identified with the use of mass spectrometry coupled with a protein profile database and tested for antibiotic susceptibility. 74% of isolated bacterial strains exhibited adaptive antibiotic resistance to at least one antibiotic. All strains were subjected to the APDT procedure using an OLED as a light source and 16 µM methylene blue as a photosensitizer. APDT using the OLED led to a large reduction in all cases. For pathogenic bacteria, the reduction ranged from 1.1-log to > 8 log (Klebsiella aerogenes, Enterobacter cloaca, Staphylococcus hominis) even for high antibiotic resistance (MRSA 5-log reduction). Opportunistic bacteria showed a range from 0.4-log reduction for Citrobacter koseri to > 8 log reduction for Kocuria rhizophila. These results show that OLED-driven APDT is effective against pathogens and opportunistic bacteria regardless of drug resistance.