Photothermal absorption spectroscopy of individual semiconductor nanocrystals

Single 5-nm quantum dot detection via microtoroid optical resonator photothermal microscopy

Label-free detection techniques for single particles and molecules play an important role in basic science, disease diagnostics, and nanomaterial investigations. While fluorescence-based methods are tools for single molecule detection and imaging, they are limited by available molecular probes and photoblinking and photobleaching. Photothermal microscopy has emerged as a label-free imaging technique capable of detecting individual nanoabsorbers with high sensitivity. Whispering gallery mode (WGM) microresonators can confine light in a small volume for enhanced light-matter interaction and thus are a promising ultra-sensitive photothermal microscopy platform. Previously, microtoroid optical resonators were combined with photothermal microscopy to detect 250 nm long gold nanorods and 100 nm long polymers. Here, we combine microtoroids with photothermal microscopy to spatially detect single 5 nm diameter quantum dots (QDs) with a signal-to-noise ratio exceeding 104. Photothermal images were generated by point-by-point scanning of the pump laser. Single particle detection was confirmed for 18 nm QDs by high sensitivity fluorescence imaging and for 5 nm QDs via comparison with theory. Our system demonstrates the capability to detect a minimum heat dissipation of 0.75 pW. To achieve this, we integrated our microtoroid based photothermal microscopy setup with a low amplitude modulated pump laser and utilized the proportional-integral-derivative controller output as the photothermal signal source to reduce noise and enhance signal stability. The heat dissipation of these QDs is below that from single dye molecules. We anticipate that our work will have application in a wide variety of fields, including the biological sciences, nanotechnology, materials science, chemistry, and medicine.

Photothermal heterodyne imaging of micron-sized objects

Micron-sized dye-doped polymer beads were imaged using transmitted/reflected light microscopy and photothermal heterodyne imaging (PHI) measurements. The transmitted/reflected light images show distinct ring patterns that are attributed to diffraction effects and/or internal reflections within the beads. In the PHI experiments pump laser induced heating changes the refractive index and size of the bead, which causes changes in the diffraction pattern and internal reflections. This creates an analogous ring pattern in the PHI images. The ring pattern disappears in both the reflected light and PHI experiments when an incoherent light source is used as a probe. When the beads are imaged in an organic medium heat transfer changes the refractive index of the environment, and gives rise to a ring pattern external to the beads in the PHI images. This causes the beads to appear larger than their physical dimensions in PHI experiments. This external signal does not appear when the beads are imaged in air because the refractive index changes in air are very small.

Thousand-Fold Enhancement of Photothermal Signals in Near-Critical CO2

Photothermal (PT) microscopy has shown strong promise in imaging single absorbing nano-objects in soft matter and biological systems. PT imaging at ambient conditions usually requires a high laser power for a sensitive detection, which prevents application to light-sensitive nanoparticles. In a previous study of single gold nanoparticles, we showed that the photothermal signal can be enhanced more than 1000-fold in near-critical xenon compared to that in glycerol, a typical medium for PT detection. In this report, we show that carbon dioxide (CO₂), a much cheaper gas than xenon, can enhance PT signals in a similar way. We confine near-critical CO₂ in a thin capillary which easily withstands the high near-critical pressure (around 74 bar) and facilitates sample preparation. We also demonstrate enhancement of the magnetic circular dichroism signal of single magnetite nanoparticle clusters in supercritical CO₂. We have performed COMSOL simulations to support and explain our experimental findings.

Optical Detection and Imaging of Nonfluorescent Matter at the Single-Molecule/Particle Level

Since the first optical detection of single molecules in 1989, single-molecule spectroscopy has developed rapidly and been widely applied in many areas. However, the vast majority of matter is extremely inefficient at emitting photons in our physical world, which seriously limits the applications of optical methods based on photoluminescence. In addition to indirect detection by fluorescence labeling, many efforts have been made to directly image nonfluorescent matter at the single-particle or single-molecule level in different ways based on the absorption or scattering interaction between light and matter. Herein, we review five popular methods for imaging nonfluorescent particles/molecules, including dark-field microscopy (DFM), surface plasmon resonance microscopy (SPRM), surface enhanced Raman microscopy (SERM), interferometric scattering microscopy (iSCAT), and photothermal microscopy (PTM). After summarizing the principles and applications of these methods, we compare the advantages and disadvantages of each method and describe further potential development and applications.

Progress and perspectives in single-molecule optical spectroscopy

We review some of the progress of single-molecule optical experiments in the past 20 years and propose some perspectives for the coming years. We particularly focus on methodological advances in fluorescence, super-resolution, photothermal contrast, and interferometric scattering and briefly discuss a few of the applications. These advances have enabled the exploration of new emitters and quantum optics; the chemistry and biology of complex heterogeneous systems, nanoparticles, and plasmonics; and the detection and study of non-fluorescing and non-absorbing nano-objects. We conclude by proposing some ideas for future experiments. The field will move toward more and better signals of a broader variety of objects and toward a sharper view of the surprising complexity of the nanoscale world of single (bio-)molecules, nanoparticles, and their nano-environments.

Carbon Materials for Solar Water Evaporation and Desalination

Seawater desalination is viewed as a promising solution to world freshwater scarcity. Solar assisted desalination is proposed to overcome the high energy consumption in current desalination technologies, as it uses abundant and sustainable solar energy as the only energy input. Interfacial solar vapor generation (SVG) has attracted considerable research interest due to its high energy conversion efficiency, simple implementation, and cost-effectiveness. Among all the candidate materials for solar evaporators, carbon-based materials stand out due to their intrinsic high solar absorption, highly tunable structure, easy preparation, low cost, and earth-abundancy. In this review, the recent progress on carbon-based materials for the development of interfacial SVG is summarized. First, a brief introduction to the basic design principles of the interfacial SVG system is presented. Then, recent efforts in carbon-based solar evaporators, from artificial structures to bioinspired configurations, focusing on their structure-function relationship are highlighted. Strategies for designing antisalt-fouling desalination systems are also summarized. Last, the challenges and opportunities of carbon-based materials for solar evaporation technology are elaborated.

Localized quenching sites in MAPbI3 investigated by fluorescence and photothermal microscopy

In this work, we developed a fluorescence and photothermal microscope with extremely large scanning range and high spatial resolution. We demonstrated the capability of this instrument by simultaneously measuring the photoluminescence and photothermal signals of the CH₃NH₃PbI₃ (MAPbI₃) film. After scanning the MAPbI₃ film on the scale of centimeters, we can obtain information of both emissive and nonemissive processes with a resolution of 200 nm at any location of the large area. We can clearly see the localized photothermal signal while the photoluminescence signal is uniform. These results directly prove that the emissive recombination happens all over the materials, but the nonemissive recombination happens only at certain localized quenching sites. The fluorescence and photothermal microscope with both large scanning range and high spatial resolution can provide information of all the relaxation channels of the excitons, showing potential applications for investigation of photophysical mechanisms in photoelectric materials.

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.

Enhanced Photothermal Therapy through the In Situ Activation of a Temperature and Redox Dual-Sensitive Nanoreservoir of Triptolide

Photothermal therapy (PTT) has attracted tremendous attention due to its noninvasiveness and localized treatment advantages. However, heat shock proteins (HSPs) associated self-preservation mechanisms bestow cancer cells thermoresistance to protect them from the damage of PTT. To minimize the thermoresistance of cancer cells and improve the efficacy of PTT, an integrated on-demand nanoplatform composed of a photothermal conversion core (gold nanorod, GNR), a cargo of a HSPs inhibitor (triptolide, TPL), a mesoporous silica based nanoreservoir, and a photothermal and redox di-responsive polymer shell is developed. The nanoplatform can be enriched in the tumor site, and internalized into cancer cells, releasing the encapsulated TPL under the trigger of intracellular elevated glutathione and near-infrared laser irradiation. Ultimately, the liberated TPL could diminish thermoresistance of cancer cells by antagonizing the PTT induced heat shock response via multiple mechanisms to maximize the PTT effect for cancer treatment.

Resonant scattering-enhanced photothermal microscopy

Photothermal (PT) microscopy is currently the most efficient approach for the detection and spectroscopy of individual non-fluorescent nano-objects based solely on their absorption. The nano-objects in current PT microscopy are usually non-resonant with the probe laser light, and the PT signal is mainly generated from the interactions of the incident probe light and the heating light-induced thermal lens around the imaged object. Inspired by the high sensitivity of the scattering field from the nano-objects near optical resonance to the variation in the local refractive index, we developed a novel strategy of resonant scattering-enhanced PT microscopy where the imaged nano-objects are near-resonant with the probe laser light. We have demonstrated this by using gold nanorods (NRs) with tunable longitudinal surface plasmon resonances. The PT signal of gold NR near-resonant with the probe light showed dramatic variation in the narrow resonance wavelength range, as small as 15 nm, and the maximal amplitude of the PT signal in this range can be enhanced up to 43 times as compared with the weak PT signal of gold NR non-resonant with the probe light. Theoretical analysis indicates that the obtained strong PT signal is mainly caused by the heat-induced variation in the polarizability of gold NR. Our novel work demonstrates the first resonant scattering-enhanced PT imaging of plasmonic nanoparticles, paving the way for the development of PT microscopy with ultra-high sensitivity toward the sensing, imaging, and spectroscopy of nanoscopic objects in complex environments.

Photothermal Imaging of Individual Nano-Objects with Large Scattering Cross Sections

Photothermal (PT) microscopy enables the efficient detection of absorbing nano-objects with high sensitivity and stability. The PT signal in the current PT microscopy usually comes from the interaction of the probe laser beam with the heating laser beam-induced thermal lens, and the contribution of the scattering field from the imaged nano-object is usually not taken into account. Here, in this paper, we systematically studied the influence of the scattering field from the imaged nanoparticles on the obtained PT signal by using Ag nanowires (NWs) on a glass substrate surrounded by glycerol as an example. Under the excitation of a heating laser beam at 532 nm wavelength, the rise of local temperature around the Ag NW results in the intensity variation of the interferometric scattering probe light at 730 nm wavelength which includes the scattering light from the Ag NW and the reflection light from the glass-glycerol interface. We found that the PT signal on the NW are positive and negative for the probe beam polarized parallel and perpendicular to the NW axis, respectively. Numerical simulations confirm that the heat-induced intensity variation of the pure scattering light from the NW and the thermal lens-induced intensity increase of the reflection light both contribute to the obtained PT signal. Our work provides the basic guidance for the analysis of PT signal from nano-objects with large scattering cross sections.

Self-assembled CeVO4/Ag nanohybrid as photoconversion agents with enhanced solar-driven photocatalysis and NIR-responsive photothermal/photodynamic synergistic therapy performance

The plasmonic cerium vanadate (CeVO₄) semiconductor and plasmonic silver (Ag) metal exhibit a localized surface plasmon resonance (LSPR) effect in the visible (Vis)-light region; however, weak absorption in the near-infrared (NIR) region restricts their environmental remediation and biomedical application. Herein, CeVO₄/Ag nanohybrids with self-assembled heterostructure and improved Vis/NIR light absorption were synthesized from CeVO₄ nanosheets and AgNO₃ solution, which could serve as potential solar-driven catalytic agents and near-infrared (NIR) light responsive anticancer agents. Oleic acid-stabilized CeVO₄ nanosheets were modified with the HS-PEG₁₀₀₀-OH by the thiol-ene click reaction and presented self-assembly morphology in aqueous solution due to hydrophobic-hydrophobic interactions. Sulfhydryl (-SH) groups provided stable sites for Ag⁺ ions on the surface of CeVO₄, and Ag⁺ ions could be directly reduced by Ce³⁺ ions to form CeVO₄/Ag heterojunction nanocrystals (NCs). Due to the higher absorption in the Vis/NIR light region than CeVO₄ nanosheets, CeVO₄/Ag NCs led to the improved solar light responsive photocatalytic degradation of organic dyes. Upon the exposure of these NCs to an 808 nm laser, CeVO₄/Ag NCs show high photothermal conversion efficiency, ROS generation ability and photoacoustic (PA) signal for implementing PA imaging-guided photothermal/photodynamic synergistic cancer therapy with better tumor inhibition effect.

Imaging and Spectroscopy of Single Metal Nanostructure Absorption

The highly tunable optical properties of metal nanoparticles make them an ideal building block in any application that requires control over light, heat, or electrons on the nanoscale. Because of their size, metal nanoparticles both absorb and scatter light efficiently. Consequently, improving their performance often involves shifting the balance between absorption and scattering to promote desirable features of their optical properties. Scattering by single metal nanoparticles is commonly characterized using dark-field scattering spectroscopy, but routine methods to characterize pure absorption over a broad wavelength range are much more complex. This article reviews work from our lab using photothermal imaging in combination with dark-field scattering and electron microscopy to separate radiative and nonradiative properties of single nanoparticles and their assemblies. We present both initial work using different laser wavelengths to explore pure absorption free from scattering contributions based on the heat released into the environment as well as the development of photothermal spectroscopy over a broad wavelength range, making it possible to resolve details that are otherwise hidden in ensemble measurements that most of the time also do not separate radiative and nonradiative properties.

Imaging the chemical activity of single nanoparticles with optical microscopy

Chemical activity of single nanoparticles can be imaged and determined by monitoring the optical signal of each individual during chemical reactions with advanced optical microscopes. It allows for clarifying the functional heterogeneity among individuals, and for uncovering the microscopic reaction mechanisms and kinetics that could otherwise be averaged out in ensemble measurements.

Direct visualization of carbon nanotube degradation in primary cells by photothermal imaging

Assessment of biodegradability of carbon nanotubes (CNTs) is a critically important aspect that needs to be solved before their translation into new biomedical tools. CNT biodegradation has been shown both in vitro and in vivo, but we are limited by the number of analytical techniques that can be used to follow the entire process. Photothermal imaging (PhI) is an innovative technique that enables the quantitative detection of nanometer-sized absorptive objects. In this study, we demonstrate that PhI allows the observation of the degradation process of functionalized multi-walled carbon nanotubes (MWCNTs) following their internalization by primary glial cells. The absence of interference from the biological matrix components, together with the possibility to combine PhI with other detection techniques (e.g. fluorescence, light or electron microscopy) validate the potential of this method to follow the fate and behavior of carbon nanostructures in a biological environment.

Small Gold Nanorods with Tunable Absorption for Photothermal Microscopy in Cells

The synthesis, sorting, and characterization of monodisperse gold nanorods with dimensions around 10 nm in length and below 6 nm in diameter is reported. They display tunable plasmon resonance in the near infrared, a region where cellular absorption is reduced. A dual color photothermal microscope is developed to demonstrate that they are promising single molecule probes for bioimaging.

Metal Nanomaterial Toxicity Variations Within the Vascular System

Engineered nanomaterials (ENM) are anthropogenic materials with at least one dimension less than 100 nm. Their ubiquitous employment in biomedical and industrial applications in the absence of full toxicological assessments raises significant concerns over their safety on human health. This is a significant concern, especially for metal and metal oxide ENM as they may possess the greatest potential to impair human health. A large body of literature has developed that reflects adverse systemic effects associated with exposure to these materials, but an integrated mechanistic framework for how ENM exposure influences morbidity remains elusive. This may be due in large part to the tremendous diversity of existing ENM and the rate at which novel ENM are produced. In this review, the influence of specific ENM physicochemical characteristics and hemodynamic factors on cardiovascular toxicity is discussed. Additionally, the toxicity of metallic and metal oxide ENM is presented in the context of the cardiovascular system and its discrete anatomical and functional components. Finally, future directions and understudied topics are presented. While it is clear that the nanotechnology boom has increased our interest in ENM toxicity, it is also evident that the field of cardiovascular nanotoxicology remains in its infancy and continued, expansive research is necessary in order to determine the mechanisms via which ENM exposure contributes to cardiovascular morbidity.

Coating Carbon Nanosphere with Patchy Gold for Production of Highly Efficient Photothermal Agent

Gold- or carbon-based photothermal therapy (PTT) agents have shown encouraging therapeutic effects of PTT in the near-infrared region (NIR) in many preclinical animal experiments. It is expected that gold/carbon hybrid nanomaterial will possess combinational NIR light absorption and can achieve further improvement in photothermal conversion efficiency. In this work, we design and construct a novel PTT agent by coating a carbon nanosphere with patchy gold. To synthesize this composite particle with Janus structure, a new versatile approach based on a facile adsorption-reduction method was presented. Different from the conventional fabrication procedures, the formation of patchy gold in this approach is mainly a thermodynamics-driven spontaneous process. The results show that when compared with the conventional PTT agent gold nanorod the obtained nanocomposites not only have higher photothermal conversion efficiency but also perform more thermally stable. On the basis of these outstanding photothermal effects, the in vitro and in vivo photothermal performances in a MCF-7 cells (human breast adenocarcinoma cell line) and mice were investigated separately. Additionally, to further illustrate the advantage of this asymmetric structure, their potential was explored by selective surface functionalization, taking advantage of the affinity of both patchy gold and carbon domain to different functional molecules. These results suggest that this new hybrid nanomaterial can be used as an effective PTT agent for cancer treatment in the future.

Hundreds-fold Sensitivity Enhancement of Photothermal Microscopy in Near-Critical Xenon

Photothermal absorption microscopy of single Au nanoparticles was conducted at temperatures and pressures near the critical point of Xenon (Tc = 16.583 °C, Pc = 5.842 MPa). The divergence of the thermal expansion coefficient at the critical point makes the refractive index highly sensitive to changes in temperature, which directly translates to a large enhancement of the photothermal signal. We find that measurements taken near the critical point of Xe give a signal enhancement factor of up to 440 ± 130 over those taken in glycerol. The highest sensitivity recorded here corresponds to power dissipation of 64 pW, achieving a signal-to-noise ratio of 9.4 for 5 nm Au nanoparticles with an integration time of 50 ms, making this the most sensitive of any absorption microscopy technique reported to date. Enhancing the sensitivity of absorption microscopy lowers the operating heating power, allowing the technique to be more compatible with absorbers with absorption coefficient and photochemical stability lower than that of Au.

Quasi first-order Hermite Gaussian beam for enhanced sensitivity in Sagnac interferometer photothermal deflection spectroscopy

The detection sensitivity of a Sagnac interferometer photothermal deflection spectroscopy was enhanced by changing the probe beam pattern from zero-order to a quasi-first-order Hermite Gaussian (QHG) beam. The nature of the higher order HG mode, where the beam pattern is preserved during propagation with an increased field gradient, is utilized to enhance the measurement sensitivity. In this spectroscopy, the lateral beam deflection due to the photothermal effect is sensitively detected as a change in the interference light intensity. The change in intensity is amplified due to the higher field gradient of the QHG(1,0) beam at the photodetector. This amplification effect was both numerically and experimentally demonstrated to obtain twofold improvement in the signal-to-noise ratio.

Optical detection of individual ultra-short carbon nanotubes enables their length characterization down to 10 nm

Ultrashort single-walled carbon nanotubes, i.e. with length below ~30 nm, display length-dependent physical, chemical and biological properties that are attractive for the development of novel nanodevices and nanomaterials. Whether fundamental or applicative, such developments require that ultrashort nanotube lengths can be routinely and reliably characterized with high statistical data for high-quality sample production. However, no methods currently fulfill these requirements. Here, we demonstrate that photothermal microscopy achieves fast and reliable optical single nanotube analysis down to ~10 nm lengths. Compared to atomic force microscopy, this method provides ultrashort nanotubes length distribution with high statistics, and neither requires specific sample preparation nor tip-dependent image analysis.

Imaging nano-objects by linear and nonlinear optical absorption microscopies

Absorption based microscopy measurements are emerging as important tools for studying nanomaterials. This review discusses the three most common techniques for performing these experiments: transient absorption microscopy, photothermal heterodyne imaging, and spatial modulation spectroscopy. The focus is on the application of these techniques to imaging and detection, using examples taken from the authors' laboratory. The advantages and disadvantages of the three methods are discussed, with an emphasis on the unique information that can be obtained from these experiments, in comparison to conventional emission or scattering based microscopy experiments.

Photothermal raster image correlation spectroscopy of gold nanoparticles in solution and on live cells

Raster image correlation spectroscopy (RICS) measures the diffusion of fluorescently labelled molecules from stacks of confocal microscopy images by analysing correlations within the image. RICS enables the observation of a greater and, thus, more representative area of a biological system as compared to other single molecule approaches. Photothermal microscopy of gold nanoparticles allows long-term imaging of the same labelled molecules without photobleaching. Here, we implement RICS analysis on a photothermal microscope. The imaging of single gold nanoparticles at pixel dwell times short enough for RICS (60 μs) with a piezo-driven photothermal heterodyne microscope is demonstrated (photothermal raster image correlation spectroscopy, PhRICS). As a proof of principle, PhRICS is used to measure the diffusion coefficient of gold nanoparticles in glycerol : water solutions. The diffusion coefficients of the nanoparticles measured by PhRICS are consistent with their size, determined by transmission electron microscopy. PhRICS was then used to probe the diffusion speed of gold nanoparticle-labelled fibroblast growth factor 2 (FGF2) bound to heparan sulfate in the pericellular matrix of live fibroblast cells. The data are consistent with previous single nanoparticle tracking studies of the diffusion of FGF2 on these cells. Importantly, the data reveal faster FGF2 movement, previously inaccessible by photothermal tracking, and suggest that inhomogeneity in the distribution of bound FGF2 is dynamic.

Single-particle absorption spectroscopy by photothermal contrast

Removing effects of sample heterogeneity through single-molecule and single-particle techniques has advanced many fields. While background free luminescence and scattering spectroscopy is widely used, recording the absorption spectrum only is rather difficult. Here we present an approach capable of recording pure absorption spectra of individual nanostructures. We demonstrate the implementation of single-particle absorption spectroscopy on strongly scattering plasmonic nanoparticles by combining photothermal microscopy with a supercontinuum laser and an innovative calibration procedure that accounts for chromatic aberrations and wavelength-dependent excitation powers. Comparison of the absorption spectra to the scattering spectra of the same individual gold nanoparticles reveals the blueshift of the absorption spectra, as predicted by Mie theory but previously not detectable in extinction measurements that measure the sum of absorption and scattering. By covering a wavelength range of 300 nm, we are furthermore able to record absorption spectra of single gold nanorods with different aspect ratios. We find that the spectral shift between absorption and scattering for the longitudinal plasmon resonance decreases as a function of nanorod aspect ratio, which is in agreement with simulations.

Photothermal microscopy: optical detection of small absorbers in scattering environments

Photothermal microscopy enables detection of nanometer-sized objects solely based on their absorption. This technique allows efficient observation of various nano-objects in scattering media notably gold nanoparticles in cells. The extreme sensitivity of the method and the stability of the signals open numerous applications in spectroscopy, analytical chemistry and bioimaging. This review briefly describes the principle and the main characteristics of photothermal microscopy, with its major advantages and limitations, and exposes the principal applications that have been carried out since its first implementation.

PEGylated carbon nanoparticles for efficient in vitro photothermal cancer therapy

PEGylated carbon nanoparticles possess strong heat-producing ability and exhibit great potential in photothermal cancer therapy.

Gaussian beam photothermal single particle microscopy

We explore the intuitive lensing picture of laser-heated nanoparticles occurring in single particle photothermal (PT) microscopy. The effective focal length of the thermal lens (TL) is derived from a ray-optics treatment and used to transform the probing focused Gaussian beam with ABCD Gaussian matrix optics. The relative PT signal is obtained from the relative beam-waist change far from the TL. The analytical expression is semiquantitative, capable of describing the entire phenomenology of single particle PT microscopy, and shows that the signal is the product of the point-spread functions of the involved lasers times a linear function of the axial coordinate. The presented particularly simple and intuitive Gaussian beam lensing picture compares favorably to the experimental results for 60 nm gold nanoparticles and provides the prescription for optimum setup calibration.

Transport of fibroblast growth factor 2 in the pericellular matrix is controlled by the spatial distribution of its binding sites in heparan sulfate

The heparan sulfate (HS) chains of proteoglycans are a key regulatory component of the extracellular matrices of animal cells, including the pericellular matrix around the plasma membrane. In these matrices they regulate transport, gradient formation, and effector functions of over 400 proteins central to cell communication. HS from different matrices differs in its selectivity for its protein partners. However, there has been no direct test of how HS in the matrix regulates the transport of its partner proteins. We address this issue by single molecule imaging and tracking in fibroblast pericellular matrix of fibroblast growth factor 2 (FGF2), stoichiometrically labelled with small gold nanoparticles. Transmission electron microscopy and photothermal heterodyne imaging (PHI) show that the spatial distribution of the HS-binding sites for FGF2 in the pericellular matrix is heterogeneous over length scales ranging from 22 nm to several µm. Tracking of individual FGF2 by PHI in the pericellular matrix of living cells demonstrates that they undergo five distinct types of motion. They spend much of their time in confined motion (∼110 nm diameter), but they are not trapped and can escape by simple diffusion, which may be slow, fast, or directed. These substantial translocations (µm) cover distances far greater than the length of a single HS chain. Similar molecular motion persists in fixed cells, where the movement of membrane PGs is impeded. We conclude that FGF2 moves within the pericellular matrix by translocating from one HS-binding site to another. The binding sites on HS chains form non-random, heterogeneous networks. These promote FGF2 confinement or substantial translocation depending on their spatial organisation. We propose that this spatial organisation, coupled to the relative selectivity and the availability of HS-binding sites, determines the transport of FGF2 in matrices. Similar mechanisms are likely to underpin the movement of many other HS-binding effectors.

Sagnac interferometer for photothermal deflection spectroscopy

Photothermal deflection spectroscopy is combined with a Sagnac interferometer to enhance the sensitivity of the absorption measurement by converting the photothermal beam deflection effect into the light intensity change by the interference effect. Because of stable light interference due to the common path, the signal intensity can be amplified without increasing the noise by extending the optical path length between a sample and a photodetector. The sensitivity is further improved by the use of focusing optics and double-pass geometry. This makes photothermal deflection spectroscopy applicable to any kind of material in the whole visible region with a xenon lamp for excitation and water or air as a deflection medium.

Nanoscale Thermotropic Phase Transitions Enhancing Photothermal Microscopy Signals

The photothermal heterodyne imaging technique enables studies of individual weakly absorbing nano-objects in various environments. It uses a photoinduced change in the refractive index of the environment. Taking advantage of the dramatic index of refraction change occurring around a thermotropic liquid-crystalline phase transition, we demonstrate a 40-fold signal-to-noise ratio enhancement for gold nanoparticles imaged in 4-cyano-4'-pentylbiphenyl (5CB) liquid crystals over those in a water environment. We studied the photothermal signal as a function of probe laser polarization, heating power, and sample temperature quantifying the optimal enhancement. This study established photothermal microscopy as a valuable technique for inducing and/or detecting local phase transitions at the nanometer scales.

Photothermal single-particle microscopy: detection of a nanolens

Combining quantitative photothermal microscopy and light scattering microscopy as well as accurate MIE scattering calculations on single gold nanoparticles, we reveal that the mechanism of photothermal single-molecule/particle detection is quantitatively explained by a nanolensing effect. The lensing action is the result of the long-range character of the refractive index profile. It splits the focal detection volume into two regions. Our results lay the foundation for future developments and quantitative applications of single-molecule absorption microscopy.

Synthesis of Silver Nanoparticles with Monovalently Functionalized Self-Assembled Monolayers

The importance of having nanoparticles that are soluble, stable, and that have no non-specific binding is often overlooked, but essential for their use in biology. This is particularly prominent with silver nanoparticles that are susceptible to the effects of aggregation and metal-surface reactivity. Here we use a combination of several small peptidols and short alkanethiol ethylene glycol ligands to develop a ligand shell that is reasonably resistant to ligand exchange and non-specific binding to groups common in biological molecules. The stability of the nanoparticles is not affected by the inclusion of a functional ligand, which is done in the same preparative step. The stoichiometry of the nanoparticles is controlled, such that monofunctional silver nanoparticles can be obtained. Two different sets of nanoparticles, functionalized with either Tris-nitrilotriacetic acid or a hexa-histidine peptide sequence, readily form dimers/oligomers, depending on their stoichiometry of functionalization.

Photothermal imaging and measurement of protein shell stoichiometry of single HIV-1 Gag virus-like nanoparticles

Virus life stages often constitute a complex chain of events, difficult to track in vivo and in real-time. Challenges are associated with spatial and time limitations of current probes: most viruses are smaller than the diffraction limit of optical microscopes while the entire time scale of virus dynamics spans over 8 orders of magnitude. Thus, virus processes such as entry, disassembly, and egress have generally remained poorly understood. Here we discuss photothermal heterodyne imaging (PHI) as a possible alternative to fluorescence microscopy in the study of single virus-like nanoparticle (VNP) dynamics, with relevance in particular to virus uncoating. Being based on optical absorption rather than emission, PHI could potentially surpass some of the current limitations associated with fluorescent labels. As proof-of-principle, single VNPs self-assembled from 60 nm DNA-functionalized gold nanoparticles (DNA-Au NPs) encapsulated in a Gag protein shell of the human immunodeficiency virus (HIV-1) were imaged, and their photothermal response was compared with DNA-Au NPs. For the first time, the protein stoichiometry of a single virus-like particle was estimated by a method other than electron microscopy.

Inorganic nanoparticles for multimodal molecular imaging

Multimodal molecular imaging can offer a synergistic improvement of diagnostic ability over a single imaging modality. Recent development of hybrid imaging systems has profoundly impacted the pool of available multimodal imaging probes. In particular, much interest has been focused on biocompatible, inorganic nanoparticle-based multimodal probes. Inorganic nanoparticles offer exceptional advantages to the field of multimodal imaging owing to their unique characteristics, such as nanometer dimensions, tunable imaging properties, and multifunctionality. Nanoparticles mainly based on iron oxide, quantum dots, gold, and silica have been applied to various imaging modalities to characterize and image specific biologic processes on a molecular level. A combination of nanoparticles and other materials such as biomolecules, polymers, and radiometals continue to increase functionality for in vivo multimodal imaging and therapeutic agents. In this review, we discuss the unique concepts, characteristics, and applications of the various multimodal imaging probes based on inorganic nanoparticles.

Absolute optical extinction measurements of single nano-objects by spatial modulation spectroscopy using a white lamp

This article describes a high sensitivity spectrophotometer designed to detect the overall extinction of light by a single nanoparticle (NP) in the 10(-4)-10(-5) relative range, using a transmission measurement configuration. We focus here on the simple and low cost scheme where a white lamp is used as a light source, permitting easy and broadband extinction measurements (300-900 nm). Using a microscope, in a confocal geometry, an increased sensitivity is reached thanks to a modulation of the NP position under the light spot combined with lock-in detection. Moreover, it is shown that this technique gives access to the absolute extinction cross-sections of the single NP provided that the incident electromagnetic field distribution experienced by the NP is accurately characterized. In this respect, an experimental procedure to characterize the light spot profile in the focal plane, using a reference NP as a probe, is also laid out. The validity of this approach is discussed and confirmed by comparing experimental intensity distributions to theoretical calculations taking into account the vector character of the tightly focused beam. The calibration procedure permitting to obtain the absolute extinction cross-section of the probed NP is then fully described. Finally, the force of the present technique is illustrated through selected examples concerning spherical and slightly elongated gold and silver NPs. Absolute extinction measurements are found to be in good consistency with the NP size and shape independently obtained from transmission electron microscopy, showing that spatial modulation spectroscopy is a powerful tool to get an optical fingerprint of the NP.

Gold nanoparticles delivery in mammalian live cells: a critical review

Functional nanomaterials have recently attracted strong interest from the biology community, not only as potential drug delivery vehicles or diagnostic tools, but also as optical nanomaterials. This is illustrated by the explosion of publications in the field with more than 2,000 publications in the last 2 years (4,000 papers since 2000; from ISI Web of Knowledge, 'nanoparticle and cell' hit). Such a publication boom in this novel interdisciplinary field has resulted in papers of unequal standard, partly because it is challenging to assemble the required expertise in chemistry, physics, and biology in a single team. As an extreme example, several papers published in physical chemistry journals claim intracellular delivery of nanoparticles, but show pictures of cells that are, to the expert biologist, evidently dead (and therefore permeable). To attain proper cellular applications using nanomaterials, it is critical not only to achieve efficient delivery in healthy cells, but also to control the intracellular availability and the fate of the nanomaterial. This is still an open challenge that will only be met by innovative delivery methods combined with rigorous and quantitative characterization of the uptake and the fate of the nanoparticles. This review mainly focuses on gold nanoparticles and discusses the various approaches to nanoparticle delivery, including surface chemical modifications and several methods used to facilitate cellular uptake and endosomal escape. We will also review the main detection methods and how their optimum use can inform about intracellular localization, efficiency of delivery, and integrity of the surface capping.

Experimental determination of single CdSe nanowire absorption cross sections through photothermal imaging

Absorption cross sections ((sigma)abs) of single branched CdSe nanowires (NWs) have been measured by photothermal heterodyne imaging (PHI). Specifically, PHI signals from isolated gold nanoparticles (NPs) with known cross sections were compared to those of individual CdSe NWs excited at 532 nm. This allowed us to determine average NW absorption cross sections at 532 nm of (sigma)abs = (3.17 +/- 0.44) x 10(-11) cm2/microm (standard error reported). This agrees well with a theoretical value obtained using a classical electromagnetic analysis ((sigma)abs = 5.00 x 10(-11) cm2/microm) and also with prior ensemble estimates. Furthermore, NWs exhibit significant absorption polarization sensitivities consistent with prior NW excitation polarization anisotropy measurements. This has enabled additional estimates of the absorption cross section parallel ((sigma)abs) and perpendicular ((sigma)abs(perpendicular) to the NW growth axis, as well as the corresponding NW absorption anisotropy ((rho)abs). Resulting values of (sigma)abs = (5.6 +/- 1.1) x 10(-11) cm2/microm, (sigma)abs(perpendicular) = (1.26 +/- 0.21) x 10(-11) cm2/microm, and (rho)abs = 0.63+/- 0.04 (standard errors reported) are again in good agreement with theoretical predictions. These measurements all indicate sizable NW absorption cross sections and ultimately suggest the possibility of future direct single NW absorption studies.

A common mechanism underlies the dark fraction formation and fluorescence blinking of quantum dots

CdSe quantum dots (QDs) are known to exhibit both power-law blinking dynamics and a dark fraction. A complete description of the mechanistic origins of these properties is still lacking. We show that a change in the pH of the QD environment systematically changes both the dark fraction and the blinking statistics. As pH is lowered, shorter "on" times and longer "off" times, as well as an increase in the permanent dark fraction, are observed. The increase in the dark fraction is preceded by a decrease in the emission intensity of a single QD. Interestingly, the form of the probability distribution function describing blinking changes when the QDs are taken from an air-exposed environment into an aqueous one. These results are used to propose a coupled role for H(+) ions by which they first reduce the intensity of the emitting state as well as affect the probabilities of the QD to switch between "on" and "off" states and eventually trap the QD in a permanent "off" state. We discuss and extend two theoretical blinking models to account for the effect of H(+) ions as well as to highlight their common principle of a diffusion-controlled mechanism governing blinking.

Photothermal absorption correlation spectroscopy

Fluorescence correlation spectroscopy (FCS) is a popular technique, complementary to cell imaging for the investigation of dynamic processes in living cells. Based on fluorescence, this single molecule method suffers from artifacts originating from the poor fluorophore photophysics: photobleaching, blinking, and saturation. To circumvent these limitations we present here a new correlation method called photothermal absorption correlation spectroscopy (PhACS) which relies on the absorption properties of tiny nano-objects. PhACS is based on the photothermal heterodyne detection technique and measures akin FCS, the time correlation function of the detected signals. Application of this technique to the precise determination of the hydrodynamic sizes of different functionalized gold nanoparticles are presented, highlighting the potential of this method.

Quantum dots as multimodal photoacoustic and photothermal contrast agents

Quantum dots (QDs) have primarily been developed as fluorescent probes with unique optical properties. We herein demonstrate an extension of these QD utilities to photoacoustic (PA) and photothermal (PT) microscopy, using a nanosecond pulse laser excitation (420-900 nm, 8 ns, 10(-3)-10 J/cm(2)). The laser-induced PA, PT and accompanying bubble formation phenomena were studied with an advanced multifunctional microscope, which integrates fluorescence, PA, PT imaging, and PT thermolens modules. It was demonstrated that QDs, in addition to being excellent fluorescent probes, can be used as PA and PT contrast agents and sensitizers, thereby providing an opportunity for multimodal high resolution (300 nm) PA-PT-fluorescent imaging as well as PT therapy. Further improvements for this technology are suggested by increasing the conversion of laser energy in PT, PA, and bubble phenomena in hybrid multilayer QDs that have optimized absorption, thermal, and acoustic properties.