Two-photon Luminescence Research Articles

Nanoscale optical properties of metal nanoparticles probed by Second Harmonic Generation microscopy

We report spatial and vectorial imaging of local fields' confinement properties in metal nanoparticles with branched shapes, using Second Harmonic Generation (SHG) microscopy. Taking advantage of the coherent nature of this nonlinear process, the technique provides a direct evidence of the coupling between the excitation polarization and both localization and polarization specificities of local fields at the sub-diffraction scale. These combined features, which are governed by the nanoparticles' symmetry, are not accessible using other contrasts such as linear optical techniques or two-photon luminescence.

A multiple-labelling method for cells using Au nanoparticles with different shapes

An important and difficult issue is simultaneously identifying the detailed locations of various molecules on the cell surface, as this identification requires a synergistic effect between more than one molecule in a living cell. Au nanoparticles (NPs) with different shapes can be readily recognised under low vacuum scanning electron microscopy (lvSEM). Anisotropic Au nanorods (NRs) possess unique surface plasmon resonance (SPR) properties, which can be further utilised for two photon luminescence (TPL) and other optical imaging techniques. In this paper, Au NRs and Au nanooctahedra (Au NOs) are introduced as biomarkers for ICAM-1 and Integrin β1. Combined with the advantages of lvSEM, this multiple-labelling method is a new method for studying the interactions between specific, functional molecules.

Two-photon luminescence properties of gold nanorods

Gold nanorods can be internalized by macrophages (an important early cellular marker in atherosclerosis and cancer) and used as an imaging contrast agent for macrophage targeting. Objective of this study is to compare two-photon luminescence (TPL) properties of four aspect ratios of gold nanorods with surface plasmon resonance at 700, 756, 844 and 1060 nm respectively. TPL from single nanorods and Rhodamine 6G particles was measured using a laser-scanning TPL microscope. Nanorod TPL emission spectrum was recorded by a spectrometer. Quadratic dependence of luminescence intensity on excitation power (confirming a TPL process) was observed below a threshold (e.g., <1.6 mW), followed by photobleaching at higher power levels. Dependence of nanorod TPL intensity on excitation wavelength indicated that the two-photon action cross section (TPACS) is plasmon-enhanced. Largest TPACS of a single nanorod (12271 GM) was substantially larger than a single Rhodamine 6G particle (25 GM) at 760 nm excitation. Characteristics of nanorod TPL emission spectrum can be explained by plasmon-enhanced interband transition of gold. Comparison results of TPL brightness, TPACS and emission spectrum of nanorods can guide selection of optimal contrast agent for selected imaging applications.

Tailoring and imaging the plasmonic local density of states in crystalline nanoprisms

Surface plasmon (SP) technologies exploit the spectral and spatial properties of collective electronic oscillations in noble metals placed in an incident optical field. Yet the SP local density of states (LDOS), which rule the energy transducing phenomena between the SP and the electromagnetic field, is much less exploited. Here, we use two-photon luminescence (TPL) microscopy to reveal the SP-LDOS in thin single-crystalline triangular gold nanoprisms produced by a quantitative one-pot synthesis at room temperature. Variations of the polarization and the wavelength of the incident light redistribute the TPL intensity into two-dimensional plasmonic resonator patterns that are faithfully reproduced by theoretical simulations. We demonstrate that experimental TPL maps can be considered as the convolution of the SP-LDOS with the diffraction-limited Gaussian light beam. Finally, the SP modal distribution is tuned by the spatial coupling of nanoprisms, thus allowing a new modal design of plasmonic information processing devices.

Plasmons rock in metal bands

Two-photon luminescence in metallic nanostructures provides a unique signature of the number of plasmonic modes per unit energy and volume, paving the way for more efficient plasmonic sources, detectors and sensors.

Two-Photon-Induced Blue Shift of Core and Shell Optical Transitions in Colloidal CdSe/CdS Quasi-Type II Quantum Rods

The spectral dependence of the two-photon absorption in CdSe/CdS core/shell nanocrystal heterorods has been studied via two-photon-induced luminescence excitation spectroscopy. We verified that the two-photon absorption in these samples is a purely nonlinear phenomenon, excluding the contribution from multistep linear absorption mediated by defect states. A large absorption cross section was observed for CdSe/CdS core/shell quantum rods, in the range of 10(5) GM (1 GM = 10(-50) cm(4) s phot(-1)), scaling with the total nanocrystal volume and thus independent of the core emission wavelength. In the two-photon luminescence excitation spectra, peaks are strongly blue-shifted with respect to the one-photon absorption peaks, for both core and shell transitions. The experimental results are confirmed by k·p calculations, which attribute the shift to both different parity selection rules that apply to one-photon and two-photon transitions and a low oscillator strength for two-photon transitions close to the ground-state one-photon absorption. In contrast with lead chalcogenide quantum dots, we found no evidence of a breakdown of the optical selection rules, despite the presence of band anisotropy, via the anisotropic hole masses, and the explicitly induced reduction of the electron wave function symmetry via the rod shape of the shell. The anisotropy does lead to an unexpected splitting of the electron P-states in the case of a large CdSe core encapsulated in a thin CdS shell. Hence, tuning of the core and shell dimensions and the concurrent transition from type I to quasi-type II carrier localization enables unprecedented control over the band-edge two-photon absorption.

Parallel Nanometric 3D Tracking of Intracellular Gold Nanorods Using Multifocal Two-Photon Microscopy

We report a novel technique for long-term parallel three dimensional (3D)-tracking of gold nanorods in live cells with nanometer resolution. Gold nanorods feature a strong plasmon-enhanced two-photon luminescence, can be easily functionalized, and have been shown to be nontoxic. These properties make gold nanorods very suitable for in vivo two-photon luminescence microscopy. By rapid multifocal scanning, we combine the advantages of 3D molecular tracking methods using wide-field imaging with the advantages of two-photon microscopy. Isolated gold nanorods can be localized with a resolution of 4 nm in the xy-plane and 8 nm in the z-direction. The polarization-dependence of the two-photon luminescence signal can be used to resolve the angular orientation, even when two gold nanorods are separated by less than the diffraction limit. Individual nanorods in live U2OS cells could be followed in 3 dimensions for over 30 min, with a photon noise limited accuracy, and a time resolution of 50 ms in 2D and 500 ms in 3D.

Size dependent competition between second harmonic generation and two-photon luminescence observed in gold nanoparticles

We investigate systematically the competition between the second harmonic generation (SHG) and two-photon-induced luminescence (TPL) that are simultaneously present in Au nanoparticles excited by using a femtosecond (fs) laser. For a large-sized (length ∼ 800 nm, diameter ∼ 200 nm) Au nanorod, the SHG appears to be much stronger than the TPL. However, the situation is completely reversed when the Au nanorod is fragmented into many Au nanoparticles by the fs laser. In sharp contrast, only the TPL is observed in small-sized (length ∼ 40 nm, diameter ∼ 10 nm) Au nanorods. When a number of the small-sized Au nanorods are optically trapped and fused into a large-sized Au cluster by focused fs laser light, the strong TPL is reduced while the weak SHG increases significantly. In both cases, the morphology change is characterized by scanning electron microscope. In addition, the modification of the scattering and absorption cross sections due to the morphology change is calculated by using the discrete dipole approximation method. It is revealed that SHG is dominant in the case when the scattering is much larger than the absorption. When the absorption becomes comparable to or larger than the scattering, the TPL increases dramatically and will eventually become dominant. Since the relative strengths of scattering and absorption depend strongly on the size of the Au nanoparticles, the competition between SHG and TPL is found to be size dependent.

Quintuple-modality (SERS-MRI-CT-TPL-PTT) plasmonic nanoprobe for theranostics

A unique quintuple-modality theranostic nanoprobe (QMT) is developed with gold nanostars for surface-enhanced Raman scattering (SERS), magnetic resonance imaging (MRI), computed tomography (CT), two-photon luminescence (TPL) imaging and photothermal therapy (PTT). The synthesized gold nanostars were tagged with a SERS reporter and linked with an MRI contrast agent Gd(3+). In vitro experiments demonstrated the developed QMT nanoprobe to be a potential theranostic agent for future biomedical applications.

Photoinduced luminescent carbon nanostructures with ultra-broadly tailored size ranges

Carbon nanoparticles (CNPs), hollow CNPs, nanodiamonds, and hybrid graphene spheres (HGSPs) are produced by using fs laser ablation in solution. These carbon nanostructures emit tunable photoluminescence and two-photon luminescence. The photoinduced layer-by-layer assembly of graphene nanosheets is observed to form HGSPs with tailored broadly-ranged sizes for the first time.

Gold: a versatile tool for in vivo imaging.

Imaging for diagnostics or for evaluating the efficacy of a particular drug constitutes a key challenge, and a topical area of research in nanomedicine. There has been a tremendous effort devoted to the evaluation of a variety of contrast agents, and gold nanomaterials due to their inherent and geometrically induced optical properties, have offered significant potential for in vivo imaging. The gold based nanostructures that are most commonly employed for biological imaging include nano-spheres, -rods, -shells, -cages and -stars. This feature article provides an overview of the current state of research in utilizing these gold nano-architectures in imaging, with particular emphasis on modalities such as two-photon luminescence, computed tomography, optical coherence tomography, near infrared and photoacoustic imaging.

Solution-dispersible Au nanocube dimers with greatly enhanced two-photon luminescence and SERS

We report the synthesis of 43-nm diameter Au nanocube dimers by using Ag(+) ions as competitive ligands to freeze L-cysteine-induced assembly process of the nanocubes to a desirable stage. Ascribed to the resonant interparticle coupling with an newly arising plasmon band at 710 nm and local field enhancement, the two-photon luminescence intensity of the Au nanocube dimers in solution was over 20 times stronger than that of the monomers in the wavelength range 555-620 nm. Furthermore, by coupling Raman tags onto the nanocube surface, a solution-based surface-enhanced Raman scattering (SERS) of the nanocube dimers had an enhancement factor of over 10 times compared to the isolated nanocubes. To sum up, with high stability in solution and attractive optical properties, the Au nanocube dimers have potential applications in in vivo bio-imaging and solution-based SERS.

One-photon excited luminescence of single gold particles diffusing in solution under pulsed illumination

Here we report on the visible luminescence of single gold nanospherical particles diffusing in water excited by a pulsed-laser at 488 nm. The signal studied by fluorescence correlation spectroscopy does not display the expected characteristics. The main deviation is obtained for the diffusion time that depends on the laser irradiance. Additional advanced methods of measurements have been implemented to further characterize the emission. These methods have allowed us to demonstrate that the luminescence does not blink even in the picosecond domain and that it does not photobleach either. The comparison between the signals obtained under pulsed and continuous excitations at the same wavelength suggests that the increase in the particle temperature plays a role in the non-linear increase in the luminescence intensity with the excitation power. As in the case of two-photon excited luminescence, it implies that a model describing the luminescence process for a single particle diffusing in liquid must take into account the whole system composed of the gold particle, the capping ligands and the surrounding water.

Plasmonic nanoprobes: from chemical sensing to medical diagnostics and therapy.

This article provides an overview of the development and applications of plasmonics-active nanoprobes in our laboratory for chemical sensing, medical diagnostics and therapy. Molecular Sentinel nanoprobes provide a unique tool for DNA/RNA biomarker detection both in a homogeneous solution or on a chip platform for medical diagnostics. The possibility of combining spectral selectivity and high sensitivity of the surface-enhanced Raman scattering (SERS) process with the inherent molecular specificity of nanoprobes provides an important multiplex diagnostic modality. Gold nanostars can provide an excellent multi-modality platform, combining two-photon luminescence with photothermal therapy as well as Raman imaging with photodynamic therapy. Several examples of optical detection using SERS and photonics-based treatments are presented to illustrate the usefulness and potential of the plasmonic nanoprobes for theranostics, which seamlessly combines diagnostics and therapy.

Gold nanorods as multifunctional probes in a liquid crystalline DNA matrix

We show how a single gold nanorod can serve as a multifunctional probe in an organized DNA matrix. Polarization analysis of two-photon luminescence excited with a femtosecond laser enables imaging of the orientation of a single nanorod, which reports the orientation of DNA strands. Carefully controlled photoinduced heating by the same laser is able to degrade the DNA matrix in a highly localized volume.

Multimodal optical microscopy in combination with gold nanorods for cancer cell imaging

The multimodal optical imaging technique, which utilizes nonlinear and linear optical processes, plays an important role in biological and biomedical research. As second-order nonlinear phenomenon, the two-photon luminescence (TPL) results from the nonlinear excitation of fluorescent molecules, while the second harmonic generation (SHG) depends on the second order nonlinear polarization, orientation, and noncentrosymmetric properties of molecules. In contrast, the linear resonance light scattering (RLS) occurs when the molecules are excited by a light beam with a wavelength close to their absorption bands. Since SHG, TPL, and RLS involve different kinds of optical processes, they might be used in parallel to provide complementary information about the structure and function of cells and tissues. Herein, we develop for the first time a multimodal optical microscopy with the capability of simultaneous SHG, TPL, and RLS imaging. We analyze theoretically and demonstrate experimentally the near-infrared irradiation-induced SHG, TPL, and RLS from the gold nanorods with nanometer spatial resolution. With the gold nanorods as the contrast agents, we further demonstrate the simultaneous SHG, TPL, and RLS imaging of A431 human epithelial skin cancer cells. This multimodal optical microscopy might provide a reliable and complementary approach for biological and biomedical research.

Time-of-Flight Photon Spectroscopy: A Simple Scheme To Monitor Simultaneously Spectral and Temporal Fluctuations of Emission on Single Nanoparticles

Here we report on a novel scheme for spectral analysis that exploits the wavelength dependence of the time-of-flight of a photon in a dispersive medium. This versatile and cost-effective method, named time-of-flight photon spectroscopy (TOFPS), has the major advantage of being compatible with time-correlated single-photon counting experiments. Consequently, each photon acquired during an experiment is characterized by two parameters, its absolute time of arrival and its color, respectively. As a result, the spectral and temporal fluctuations of the emission of a single nano-object can be derived from a single measurement. As a proof of the concept, we demonstrate in the paper that the method can be used to perform Raman spectroscopy as well as fluorescence spectroscopy. We emphasize that TOFPS proves to be very efficient for improving signal-to-noise ratio in fluorescence correlation spectroscopy measurements by subsequent spectral filtering and to record luminescence spectra from single metallic particles. We demonstrate that the opportunity of simultaneously recording spectral and temporal fluctuations could be used to sort particles of different shapes inside a sample. TOFPS furthermore allows developing a new type of time interval distribution analysis which correlates the time interval between two photons and their corresponding color shift. It is applied to the analysis of the two-photon excited luminescence of a single gold nanorod. This method has a potential for a broad range of applications, among which time-resolved SERS spectroscopy and analysis of the dynamics of emission processes can be handled with new statistical approaches based on the correlation of spectral and temporal fluctuations.

Nanofocusing in a metal–insulator–metal gap plasmon waveguide with a three-dimensional linear taper

The development of techniques for efficiently confining photons on the deep sub-wavelength spatial scale will revolutionize scientific research and engineering practices. The efficient coupling of light into extremely small nanofocusing devices has been a major challenge in on-chip nanophotonics because of the need to overcome various loss mechanisms and the on-chip nanofabrication challenges. Here, we demonstrate experimentally the achievement of highly efficient nanofocusing in an Au–SiO_2–Au gap plasmon waveguide using a carefully engineered three-dimensional taper. The dimensions of the SiO_2 layer, perpendicular to the direction of wave propagation, taper linearly below 100 nm. Our simulations suggest that the three-dimensional linear-tapering approach could focus 830 nm light into a 2 × 5 nm^2 area with ≤3 dB loss and an intensity enhancement of 3.0 × 10^4. In a two-photon luminescence measurement, our device achieved an intensity enhancement of 400 within a 14 × 80 nm2 area, and a transmittance of 74%.

Development of Chitosan Oligosaccharide-Modified Gold Nanorods for in Vivo Targeted Delivery and Noninvasive Imaging by NIR Irradiation

In the present study, we demonstrate the synthesis and applications of multifunctional gold nanorod-based probes for specific targeting and noninvasive imaging based on localized heating generated by gold nanorods after NIR irradiation. The structural design of the probe consists of MUA (11-mercaptoundecanoic acid)-capped gold nanorods covalently linked with low-molecular-weight chitosan oligosaccharide (M(w) ~5000) via carbodiimide (EDC) coupling agent. This surface modification is performed for complete replacement of toxic CTAB (hexadecyltrimethyl-ammonium chloride) and acid-responsive delivery of gold nanorods in acidic environment as known to be present at tumor surrounding areas. The resulting chitosan oligosaccharide-modified gold nanorods (CO-GNRs) were further conjugated with tumor targeting monoclonal antibody against EGFR (epidermal growth factor receptor) to provide localized targeting functionality owing to the overexpression of EGFR in human oral adenosquamous carcinoma cell line CAL 27. Initial in vitro and in vivo toxicity assessments indicated that CO-GNRs did not induce any significant toxicity and are thus suitable for biological applications. Furthermore, selective targeting and accumulation of CO-GNRs were observed in vitro via two-photon luminescence imaging studies in CAL 27, which was also observed through in vivo targeting studies performed via NIR (near-infrared) laser irradiation in CAL 27 xenografts of BALB/c nude mice. Hence, the CO-GNRs that we have developed are biocompatible and nontoxic and can be a potential candidate for in vivo targeted delivery, noninvasive imaging based on localized hyperthermia, and photothermal-related therapies.

Modified threshold of two-photon-pumped random lasing of ZnO nanorods by femtosecond laser ablation

We investigated systematically the modification of the two-photon-pumped random lasing (TPPRL) behavior of ZnO nanorods through femtosecond (fs) laser ablation. The excitation wavelength dependence of the TPPRL behavior of ZnO nanorods was measured and the threshold of TPPRL before and after fs laser ablation was compared. With increasing excitation wavelength, a rapid increase in the threshold of TPPRL was observed at ∼720 nm and it offers us the opportunity to significantly modify the two-photon luminescence from ZnO nanorods by fs laser ablation. The excitation wavelength dependence of the threshold was attributed to the wavelength dependence of the mean free path of photons in ZnO nanorods. When the fs laser ablation was carried out at 720 nm, it was found that the improvement in TPPRL could be achieved at excitation wavelengths longer than 620 nm while the TPPRL behavior became deteriorated for excitation wavelengths shorter than 620 nm. If the ablation and characterization wavelengths were chosen to be the same, then no obvious change was found for wavelengths shorter than 680 nm. However, a significant improvement in TPPRL could still be observed at 720 nm. The effects of ablation time and ablation method on the TPPRL behavior were also studied. It was revealed that the reduction in the threshold and the enhancement in the internal quantum efficiency could be realized by increasing ablation time and using continuous ablation. The results presented in this paper will be useful for the modification of the TPPRL behavior of random lasing media.