Insight into the photocatalytic and photothermal effect in plasmon-enhanced water oxidation property of AuTNP@MnOx core-shell nanoconstruct.

The development of robust and efficient photocatalytic constructs for boosting the water oxidation reaction (WOR) is needed for establishing a sunlight-driven renewable energy infrastructure. Here, we synthesized plasmonic core-shell nanoconstructs consisting of triangular gold nanoprism (AuTNP) core with mixed manganese oxide (MnOx) shell for photoelectrocatalytic WOR. These constructs show electrocatalytic WOR with a low onset overpotential requirement of 270 mV at pH 10. Photoexcitation showed further enhancement of their catalytic activity resulting in ∼15% decrease of the onset overpotential requirement along with the generation of photocurrent density of up to 300 µA/cm2. We showed that such light-driven enhancement of AuTNP@MnOx dyad's catalytic activity toward the WOR process includes contributions from both photocatalytic (hot carriers driven) and photothermal effects with photothermal effect playing the major role for wavelength between 532 and 808 nm. The contribution from the photocatalytic effect is appreciable only for high-energy excitations near the interband region, while the photothermal effect largely dominates for lower energy excitations near the LSPR wavelengths of the dyad.

Dog-bone shaped gold nanoparticle-mediated chemo-photothermal therapy impairs the powerhouse to trigger apoptosis in cancer cells.

The mitochondrion has emerged as one of the uncommon targets in cancer therapeutics due to its involvement in cancer generation and progression. Consequently, nanoplatform mediated delivery of anti-cancer drugs into the mitochondria of cancer tissues demonstrated immense potential in cancer treatment. In the last couple of decades, gold nanoparticles have gained incredible attention in biomedical applications due to their easy synthesis, size-shape tenability, optical properties and outstanding photothermal ability. However, application of gold nanoparticles to target mitochondria to induce the chemo-photothermal effect in cancer has remained in its infancy. To address this, herein we have engineered dog-bone shaped gold nanoparticles (Mito-AuDB-NPs) comprising cisplatin and 10-hydroxycamptothecin as chemotherapeutic drugs along with the triphenylphosphonium (TPP) cation for mitochondria homing. Mito-AuDB-NPs exhibited a remarkable increase in temperature till 56 °C upon 18 min irradiation with 740 nm NIR LED light with a power density of 0.9 W cm-2. These Mito-AuDB-NPs successfully homed into the mitochondria of HeLa cervical cancer cells within 1 h and induced mitochondrial outer membrane permeabilization (MOMP) under the chemo-photothermal effect leading to the generation of reactive oxygen species (ROS). This Mito-AuDB-NP-mediated mitochondrial damage triggered programmed cell death (apoptosis) by decreasing the expression of anti-apoptotic Bcl-2/Bcl-xl and increasing the expression of pro-apoptotic BAX followed by caspase-3 cleavage towards extraordinary HeLa cell killing in a synergistic manner without showing toxicity towards non-cancerous RPE-1 human epithelial retinal pigment cells. We anticipate that this dog-bone shaped gold nanoparticle-mediated chemo-photothermal impairment of mitochondria in the cancer cells can open a new direction towards organelle targeted cancer therapy.

Ligand-mediated electron transport channels enhance photocatalytic activity of plasmonic nanoparticles.

Photoexcitation of noble metal nanoparticles creates surface plasmons which further decay to form energetic charge carriers. These charge carriers can initiate and/or accelerate various chemical processes at nanoparticle surfaces, although the efficiency of such processes remains low as a large fraction of these carriers recombine before they can reach the reaction sites. Thus efficient utilization of these charge carriers requires designing nanostructures that promote the separation of charges and their transport toward the reaction sites. Here we demonstrate that covalently bound surface-coating ligands with suitable orbital alignment can provide electron transport channels boosting hot electron extraction from a gold nanostructure leading to a huge enhancement in the rate of hydrogen evolution reaction (HER) under NIR excitation. A (p)Br-Ph-SH substituted gold nanoprism (AuTP) substrate produced ∼4500 fold more hydrogen compared to a pristine AuTP substrate under 808 nm excitation. Further experimental and theoretical studies on a series of substituted benzene-thiol bound AuTP substrates showed that the extent of the ligand-mediated HER enhancement depends not only on the polarity of the ligand but on the interfacial orbitals interactions.

Use of Single-Molecule Plasmon-Enhanced Fluorescence to Investigate Ligand Binding to G-Quadruplex DNA.

Single-molecule measurements are crucial for studying the interactions between G-quadruplex (GQ) DNA and ligands, as they provide higher resolution and sensitivity compared to those of bulk measurements. In this study, we employed plasmon-enhanced fluorescence to investigate the real-time interaction between the cationic porphyrin ligand TmPyP4 and different topologies of telomeric GQ DNA at the single-molecule level. By analyzing the time traces of the fluorescence bursts, we extracted dwell times for the ligand. For parallel telomeric GQ DNA, the dwell time distribution followed a biexponential fit, yielding mean dwell times of 5.6 and 18.6 ms. For the antiparallel topology of human telomeric GQ DNA, plasmon-enhanced fluorescence of TmPyP4 was observed, with dwell time distributions following a single-exponential fit and a mean dwell time of 5.9 ms. Our approach allows the nuances of GQ-ligand interactions to be captured and holds promise for studying weakly emitting GQ ligands at the single-molecule level.

NIR-Driven Photocatalytic Hydrogen Production by Silane- and Tertiary Amine-Bound Plasmonic Gold Nanoprisms.

Near-infrared (NIR) photon-driven H2 production from water is regarded as one of the best routes for establishing a sustainable hydrogen-based energy economy. Here, we have developed a gold nanoprism-based photocatalytic assembly, rationally capped with an amine and a silane ligand pair, which exhibited an excellent H2 production rate (146 µL mg-1 h-1) in neutral water while achieving an absolute incident photon-to-hydrogen conversion efficiency of 0.53%. An array of spectroscopic and microscopic experiments unravel that the amine ligand scavenges the hot hole while the silane aids the H2 production via hydrolysis during the photocatalysis on the plasmon surface. This photocatalytic H2 production reactivity can be retained for multiple cycles following the replenishment of amine and silane. Hence, this photocatalytic assembly can set up the template for a large-scale NIR-driven H2 production unit.

Light-induced in situ active tuning of the LSPR of gold nanorods over 90 nm.

We demonstrate an easy and controllable method for light-induced active tuning of the longitudinal surface plasmon resonance (LSPR) of gold nanorods (AuNRs) over ∼94nm. The red-shift of the LSPR can be controlled by varying the time of exposure to a 532 nm laser. The tuning is achieved by photo-induced dissolution of individual AuNRs by sodium dodecyl sulfate (SDS) under continuous illumination. The dissolution of the AuNRs increases the aspect ratio, and consequently the LSPR exhibits a gradual but large redshift. A key feature is that it is possible to selectively tune the LSPR of a specific AuNR in a group while leaving the others totally unaffected. Such controllable, light-induced, post-synthesis fine-tuning of the LSPR is useful for tailoring the plasmonic response of individual AuNRs for a wide range of applications.

Photothermal Microscopy: Imaging the Optical Absorption of Single Nanoparticles and Single Molecules.

The photothermal (PT) signal arises from slight changes of the index of refraction in a sample due to absorption of a heating light beam. Refractive index changes are measured with a second probing beam, usually of a different color. In the past two decades, this all-optical detection method has reached the sensitivity of single particles and single molecules, which gave birth to original applications in material science and biology. PT microscopy enables shot-noise-limited detection of individual nanoabsorbers among strong scatterers and circumvents many of the limitations of fluorescence-based detection. This review describes the theoretical basis of PT microscopy, the methodological developments that improved its sensitivity toward single-nanoparticle and single-molecule imaging, and a vast number of applications to single-nanoparticle imaging and tracking in material science and in cellular biology.

Synthesis of Solution-Stable End-to-End Linked Gold Nanorod Dimers via pH-Dependent Surface Reconfiguration.

End-to-end dimers of gold nanorods are predicted to be excellent substrates for surface-enhanced spectroscopy. However, the synthesis of solution-stable end-to-end dimers remains challenging. We exploit the pH-dependent configurational change of polyelectrolytes to initiate and terminate the gold nanorod assembly formation to produce end-to-end linked dimers in high yield. The gold nanorods are first overcoated with a polyelectrolyte, and the end-to-end attachment is initiated by adding a thiol linker in acidic medium. The assembly formation is then terminated at the dimer stage by changing the pH of the medium by the addition of an appropriate amount of 1,4-diazabicyclo[2.2.2]octane (DABCO).The nanorod dimers synthesized here are stable in solution for a week without any additional surface encapsulation.

In situ modulation of gold nanorod's surface charge drives the growth of end-to-end assemblies from dimers to large networks that enhance single-molecule fluorescence by 10 000-fold.

End-to-end assemblies of anisotropic plasmonic nanostructures with small nanogaps are of great interest as they create strong hot spots for enhancing weak fluorescence and/or scattering of molecules. Here we report the growth of dithiol-linked end-to-end assemblies of gold nanorods from dimers to large networks containing thousands of individual nanorods, directed by in situ tuning of nanorod's surface charge. Surface charge was lowered to initiate the aggregation process but was subsequently increased to achieve slow tip-specific growth over seven days to form end-to-end networks of nanorods, which were stable in solution for over one month. Furthermore, we showed that these assemblies contained strong plasmonic hot spots which enhanced the fluorescence signal of a weak emitter by 104-fold. This enhancement is approximately 10-fold larger than that obtained using a single gold nanorod and is comparable to the largest enhancement obtained using more expensive lithographically made in-plane antenna arrays.

Core-shell Au@AuAg nano-peanuts for the catalytic reduction of 4-nitrophenol: critical role of hollow interior and broken shell structure.

Bimetallic hollow core-shell nanoparticles have gained immense attention, especially as a high-performance catalyst due to their large surface area and increased number of uncoordinated atoms. However, the synthesis of an anisotropic hollow structure with large number of uncoordinated atoms and tailored hole size remains elusive. Herein, we report the synthesis of peanut-like core-shell nanostructures consisting of Au nanorods as the core covered by the AuAg alloy shell. The AuAg shell was formed on the Au nanorod core via co-deposition of Ag and Au atoms without disturbing the Au nanorod core. Then, we controllably and selectively removed Ag atoms from the shell to create "Broken Shell Peanuts" with variable hole size between 8 ± 4 nm and 26 ± 7 nm. Further, we utilized these nanostructures with different hole size as catalysts to reduce 4-nitrophenol to 4-aminophenol where the broken shell peanut nanostructures with a hole size of 26 ± 7 nm were found to be 12 times more efficient than the solid shell peanut structures.

Synthesis of Complex Nanoparticle Geometries via pH-Controlled Overgrowth of Gold Nanorods.

We show that many complex gold nanostructures such as the water chestnut, dog bone, nanobar, and octahedron, which are not easily accessible via a direct seed-growth synthesis approach, can be prepared via overgrowth of the same gold nanorods by varying pH and Ag concentrations in the growth solution. Overgrown nanostructures' shapes were determined by the rate of gold atom deposition, which is faster at higher pH. In the presence of AgNO3, codeposition of gold and silver atoms affects the shapes of overgrown nanostructures, particularly at high pH.

A "turn-off" red-emitting fluorophore for nanomolar detection of heparin.

A simple fluorophore bearing a diethylaminocoumarin donor and a pyridinium acceptor was synthesized and utilized for the ultra-sensitive detection of heparin. The synthesized dicationic push-pull coumarin derivative emits strongly in the red-region (665 nm) and detects nanomolar concentrations (14.8 nM to 148 nM) of heparin in HEPES buffer and FBS serum solutions. The dication exhibits excellent fluorescence selectivity and sensitivity towards heparin over its analogues such as chondroitin 4-sulfate (CS), hyaluronic acid (HA) and dextran. This fluorescence assay is a convenient, sensitive method for monitoring heparin levels in biological samples. These findings were confirmed using coarse-grained Monte Carlo simulations, which provide us with a rationale for the selective binding of heparin.

In situ tuning of gold nanorod plasmon through oxidative cyanide etching.

Single gold nanorods exhibit great opportunities for bio-sensing, enhanced spectroscopies and photothermal therapy. A key property of these particles is the surface plasmon resonance, that is strongly dependent on their shape. Methods for tuning this resonance after the synthesis of the particles are of great interest for many applications. In this work we show that, through very well known chemistry between gold atoms and cyanide ions, it is possible to tune the surface plasmon of single 25 × 50 nm rods by more than 100 nm towards longer wavelengths. This is achieved by slowly etching gold atoms from the surface of the particles, preserving their specific optical properties.

Enhanced-fluorescence correlation spectroscopy at micro-molar dye concentration around a single gold nanorod.

Fluorescence correlation spectroscopy (FCS) is a standard tool for studying diffusion of molecules in solution, but is limited to low analyte concentrations, in the range between 10 pM and 1 nM. Such concentration limitations can be overcome by using a plasmonic nanoantenna which confines the electric field of excitation light into a tiny volume near its surface and thereby reduces the effective excitation volume by several orders of magnitude. Here we demonstrate successful FCS measurements on a 1 µM solution of crystal violet (CV) dye in glycerol using a gold nanorod antenna. Our correlation analysis yields two components: (i) a slow component with correlation time of about 100 ms, which is attributed to sticking and bleaching of the dye, and (ii) a fast component of about 1 ms, which could arise from dye diffusion through the near-field of the nanorod and/or from blinking due to intersystem crossing or photochemistry.

Resonant plasmonic enhancement of single-molecule fluorescence by individual gold nanorods.

Enhancing the fluorescence of a weak emitter is important to further extend the reach of single-molecule fluorescence imaging to many unexplored systems. Here we study fluorescence enhancement by isolated gold nanorods and explore the role of the surface plasmon resonance (SPR) on the observed enhancements. Gold nanorods can be cheaply synthesized in large volumes, yet we find similar fluorescence enhancements as literature reports on lithographically fabricated nanoparticle assemblies. The fluorescence of a weak emitter, crystal violet, can be enhanced more than 1000-fold by a single nanorod with its SPR at 629 nm excited at 633 nm. This strong enhancement results from both an excitation rate enhancement of ∼130 and an effective emission enhancement of ∼9. The fluorescence enhancement, however, decreases sharply when the SPR wavelength moves away from the excitation laser wavelength or when the SPR has only a partial overlap with the emission spectrum of the fluorophore. The reported measurements of fluorescence enhancement by 11 nanorods with varying SPR wavelengths are consistent with numerical simulations.

Probing, Sensing, and Fluorescence Enhancement with Single Gold Nanorods.

Gold nanorods with dimensions around 10-100 nm present original optical properties. Their main advantages are the tunability from 600 to 1000 nm of their main absorption band, and its high intensity, stemming from the good conducting properties of gold in this spectral range. Gold nanorods have been applied to tracking, probing, sensing, and manipulation experiments. Here, we discuss experiments done with single gold nanorods with emphasis on recent results from our group.

Toward single-molecule microscopy on a smart phone.

Thanks to fluorescence, single nano-objects down to individual fluorophores can now be imaged in optical microscopes. Fluorescence imaging is still restricted to laboratory facilities as it usually involves expensive and bulky instrumentation. A report by Wei et al. in this issue of ACS Nano, however, shows that a sensitive, cost-effective, and portable device can be developed to image individual nano-objects as small as large viruses. This work opens the fascinating prospects of single-molecule microscopy and spectroscopy on a smart phone. We speculate on the possible applications of such a portable imaging device and on the perspectives it may open in different fields of science and technology.

Detailed mechanism for the orthogonal polarization switching of gold nanorod plasmons.

In this work, we describe an electro-optic material capable of orthogonally switching the polarization of the localized surface plasmon resonance scattering of single gold nanorods, independent of their orientation. Liquid crystal samples are prepared in a sandwich configuration with electrodes arranged so that an applied voltage induces alignment-switching of the liquid crystal molecules covering individual gold nanorods. Due to the birefringence of the nematic liquid crystal, the reorientation in the nematic director alignment causes a change in the output polarization of the scattered light. We propose the underlying mechanism to be based on a homogeneous nematic to twisted nematic phase transition and provide support for it via Jones calculus by modelling the effect of ideally aligned homogeneous nematic and twisted nematic phases on polarized light transmitted through the sample. In the model, we include the effects of sample thickness and surface plasmon resonance wavelength, expressed in terms of the phase retardation, χ, on the observed output polarization. We find four distinctively different trends for the output polarization as a function of the incident polarization as χ is varied. Two of these cases provide reproducible orthogonal polarization switching of the surface plasmon resonance while maintaining a high degree of polarization. These results are verified experimentally with liquid crystal cells of different thicknesses. The deviation of the experimental samples from ideal behaviour can be explained by the inherent variations in the surface plasmon resonance maximum and local cell thickness.

Synthesis and single-molecule imaging of highly mobile adamantane-wheeled nanocars.

The synthesis and single-molecule imaging of two inherently fluorescent nanocars equipped with adamantane wheels is reported. The nanocars were imaged using 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) as the chromophore, which was rigidly incorporated into the nanocar chassis via Sonogashira cross-coupling chemistry that permitted the synthesis of nanocars having different geometries. In particular, studied here were four- and three-wheeled nanocars with adamantane wheels. It was found that, for the four-wheeled nanocar, the percentage of moving nanocars and the diffusion constant show a significant improvement over p-carborane-wheeled nanocars with the same chassis. The three-wheeled nanocar showed only limited mobility due to its geometry. These results are consistent with a requisite wheel-like rolling motion. We furthermore developed a model that relates the percentage of moving nanocars in single-molecule experiments with the diffusion constant. The excellent agreement between the model and the new results presented here as well as previous single-molecule studies of fluorescent nanocars yields an improved understanding of motion in these molecular machines.