Advances and Perspectives of Responsive Probes for Measuring γ-Glutamyl Transpeptidase

Gamma-glutamyltransferase (GGT) is a plasma-membrane-bound enzyme that is involved in the γ-glutamyl cycle, like metabolism of glutathione (GSH). This enzyme plays an important role in protecting cells from oxidative stress, thus being tested as a key biomarker for several medical conditions, such as liver injury, carcinogenesis, and tumor progression. For measuring GGT activity, a number of bioanalytical methods have emerged, such as chromatography, colorimetric, electrochemical, and luminescence analyses. Among these approaches, probes that can specifically respond to GGT are contributing significantly to measuring its activity in vitro and in vivo. This review thus aims to highlight the recent advances in the development of responsive probes for GGT measurement and their practical applications. Responsive probes for fluorescence analysis, including "off-on", near-infrared (NIR), two-photon, and ratiometric fluorescence response probes, are initially summarized, followed by discussing the advances in the development of other probes, such as bioluminescence, chemiluminescence, photoacoustic, Raman, magnetic resonance imaging (MRI), and positron emission tomography (PET). The practical applications of the responsive probes in cancer diagnosis and treatment monitoring and GGT inhibitor screening are then highlighted. Based on this information, the advantages, challenges, and prospects of responsive probe technology for GGT measurement are analyzed.

Organic Nanoparticles with Aggregation-Induced Emission and Two-Photon Excitation for Fluorescence Imaging of Living Cells/Tissues

Aggregation-induced emission (AIE) nanoparticles (NPs) have been applied in bioimaging for cancer diagnosis due to high fluorescence efficiency. However, the poor cell permeability as well as autofluorescence of biological cells/tissues caused by ultraviolet (UV) irradiation is still the key problem of AIE luminophores for biological imaging. Here, we report green-emitting organic AIE luminophores for fluorescence imaging of living cells/tissues, which possess high fluorescence quantum yields and strong AIE under two-photon excitation with near-infrared light beyond 800 nm. These AIE luminophores can bind with bovine serum albumin (BSA) to form biocompatible BSA/AIE-NPs due to their terminal aldehyde groups providing specific anchor sites with the receptor groups in BSA. Furthermore, one/two-photon fluorescence bioimaging for Hela cancer cells has been successfully carried out with BSA/AIE-NPs as the fluorescent probe, and BSA/AIE-NPs show excellent stain properties with a fast permeability of only 5 min, high cellular uptakes, and strong fluorescence. The results demonstrate the great advantages of BSA/AIE-NPs in fast fluorescence biological imaging as well as further cancer diagnosis and therapy.

Two-color Semiconductor Mode-locked Laser system for Multiphoton Imaging Applications

We demonstrate an all-semiconductor mode-locked laser system consisting of two external cavity mode-locked lasers operating at wavelengths 834 nm and 974 nm which use semiconductor optical amplifiers as gain media. The two-color laser system emits picosecond pulses with average powers of 25 mW and 60 mW resulting in peak powers exceeding 100 W and 80 W respectively. Synchronized output pulse trains from the lasers with a repetition rate of 282 MHz exhibit a relative timing jitter of 7.3 ps. Fiber coupled output from the laser system delivers an ideal output beam with TEM₀₀ mode profile. Peak power densities >1 GW/cm² can be achieved by focusing the output beam to a smaller spot with 4 μm diameter, which is crucial for applications that requires excitation of optical nonlinearities.

A computational two-photon fluorescence approach for revealing label-free the 3D image of viruses and bacteria

Current solutions for bacteria and viruses identification are based on time-consuming technics with complex preparation procedures. In the present work, we revealed label-free the presence of free viral particles and bacteria with a computational two-photon fluorescence (C-TPF) strategy. Six bacteria were tested: Escherichia coli, Staphylococcus epidermidis, Proteus vulgaris, Pseudomonas fluorescens, Bacillus subtilis, and Clostridium perfringens. The two families of viral particles were the herpes virus with the cytomegalovirus (CMV, 300 nm of diameter) and the coronavirus with the SARS-CoV-2 (100 nm of diameter). The instrumental and computational pipeline FAMOUS optimized the produced 3D images. The origin of the fluorescence emission was discussed for bacteria regarding to their two-photon excitation spectra and attributed to the metabolic indicators (FAD and NADH). The optical and computational strategy constitute a new approach for imaging label-free viral particles and bacteria and paves the way to a new understanding of viral or bacterial ways of infection.

Toward Strong Near-Infrared Absorption/Emission from Carbon Dots in Aqueous Media through Solvothermal Fusion of Large Conjugated Perylene Derivatives with Post-Surface Engineering

Carbon dots (CDs) have attracted significant interest as one of the most emerging photoluminescence (PL) nanomaterials. However, the realization of CDs with dominant near-infrared (NIR) absorption/emission peaks in aqueous solution remains a great challenge. Herein, CDs with both main NIR absorption bands at 720 nm and NIR emission bands at 745 nm in an aqueous solution are fabricated for the first time by fusing large conjugated perylene derivatives under solvothermal treatment. With post-surface engineering, the polyethyleneimine modified CDs (PEI-CDs) exhibit enhanced PL quantum yields (PLQY) up to 8.3% and 18.8% in bovine serum albumin aqueous and DMF solutions, which is the highest PLQY of CDs in NIR region under NIR excitation. Density functional theory calculations support the strategy of fusing large conjugated perylene derivatives to achieve NIR emissions from CDs. Compared to the commercial NIR dye Indocyanine green, PEI-CDs exhibit excellent photostability and much lower cost. Furthermore, the obtained PEI-CDs illustrate the advantages of remarkable two-photon NIR angiography and in vivo NIR fluorescence bioimaging. This work demonstrates a promising strategy of fusing large conjugated molecules for preparing CDs with strong NIR absorption/emission to promote their bioimaging applications.

Rational Construction of a Two-Photon NIR Ratiometric Fluorescent Probe for the Detection of Bisulfite in Live Cells, Tissues, and Foods

In this study, we report a novel ratiometric fluorescent probe with a blue shift of 180 nm based on a D-π-A-A structure. The probe composed of a hydroxyl moiety as a donor, a naphthyl ring as a π bridge, and benzothiazole/hemicyanine as an acceptor has good selectivity and high sensitivity to bisulfite (HSO₃^-) in aqueous solution. Besides one-photon fluorescence properties, the probe possesses excellent two-photon fluorescence properties and is successfully utilized for fluorescence imaging of HSO₃^- in MCF-7 cells and rat liver tissues. More importantly, the probe also has practical application potential for measuring the HSO₃^- content of real food samples.

Combined Second Harmonic Generation and Fluorescence Analyses of the Structures and Dynamics of Molecules on Lipids Using Dual-Probes: A Review

Revealing the structures and dynamic behaviors of molecules on lipids is crucial for understanding the mechanism behind the biophysical processes, such as the preparation and application of drug delivery vesicles. Second harmonic generation (SHG) has been developed as a powerful tool to investigate the molecules on various lipid membranes, benefiting from its natural property of interface selectivity, which comes from the principle of even order nonlinear optics. Fluorescence emission, which is in principle not interface selective but varies with the chemical environment where the chromophores locate, can reveal the dynamics of molecules on lipids. In this contribution, we review some examples, which are mainly from our recent works focusing on the application of combined spectroscopic methods, i.e., SHG and two-photon fluorescence (TPF), in studying the dynamic behaviors of several dyes or drugs on lipids and surfactants. This review demonstrates that molecules with both SHG and TPF efficiencies may be used as intrinsic dual-probes in plotting a clear physical picture of their own behaviors, as well as the dynamics of other molecules, on lipid membranes.

Ultrafast two-photon fluorescence imaging of cerebral blood circulation in the mouse brain in vivo

SignificanceCharacterizing blood flow by tracking individual red blood cells as they move through vessels is essential for understanding vascular function. With high spatial resolution, two-photon fluorescence microscopy is the method of choice for imaging blood flow at the cellular level. However, its application is limited to a low flow speed regimen in anesthetized animals by its slow focus scanning mechanism. Using an ultrafast scanning module, we demonstrated two-photon fluorescence imaging of blood flow at 1,000 two-dimensional frames and 1,000,000 one-dimensional line scans per second in the brains of awake mice. These ultrafast measurements enabled us to study hemodynamic and fluid mechanical regimens beyond the reach of conventional methods.

Multifunctional plasmonic gold nanostars for cancer diagnostic and therapeutic applications

Gold nanostar (AuNSt) has gained great attention in bioimaging and cancer therapy due to their tunable surface plasmon resonance across the visible-near infrared range. Photothermal treatment and imaging capabilities including fluorescence lifetime imaging at two-photon excitation (TP-FLIM) and dark-field microscopic imaging are considered in this work. Two types of AuNSts having plasmon absorption peaks centred at 600 and 750 nm wavelength were synthesized and studied. Both NSts exhibited low cytotoxicity on A549 human lung carcinoma cells. A strong emission at two-photon excitation was observed for both NSts, well-distinguishable from lifetimes of bio-object autofluorescence. High efficiency in raising the temperature in the NSts environment with the irradiation of near infrared, AuNSts triggered photothermal effect. The decreased cell viability of A549 observed via MTT test and the cell membrane damaging was demonstrated with trypan blue staining. These results suggest AuNSts can be agents with tunable plasmonic properties for imaging and photothermal therapy.

Recent Advances in Two-Photon AIEgens and Their Application in Biological Systems

Two-photon fluorescence imaging technology has the advantages of high light stability, little light damage, and high spatiotemporal resolution, which make it a powerful biological analysis method. However, due to the high concentration or aggregation state of traditional organic light-emitting molecules, the fluorescence intensity is easily reduced or disappears completely, and is not conducive to optimal application. The concept of aggregation-induced emission (AIE) provides a solution to the problem of aggregation-induced luminescence quenching (ACQ), and realizes the high fluorescence quantum yield of luminescent molecules in the aggregation state. In addition, two-photon absorption properties can readily be improved just by increasing the loading content of AIE fluorogen (AIEgen). Therefore, the design and preparation of two-photon fluorescence probes based on AIEgen to achieve high-efficiency fluorescence imaging in vitro/in vivo has become a major research hotspot. This review aims to summarize representative two-photon AIEgens based on triphenylamine, tetraphenylethene, quinoline, naphthalene and other new structures from the past five years, and discuss their great potential in bioimaging applications.

In Vivo Bioimaging and Photodynamic Therapy Based on Two-Photon Fluorescent Conjugated Polymers Containing Dibenzothiophene-S,S-dioxide Derivatives

As a critical component for photodynamic therapy toward cancer treatment, photosensitizers require high photoinduced reactive oxygen species generation efficiency, good biocompatibility, and high phototoxicity. Herein, a series of donor-acceptor conjugated polymers containing dibenzothiophene-S,S-dioxide derivatives are designed and synthesized, which can be used as effective photosensitizers. The resulting copolymer PTA5 shows strong green light emission with high photoluminescence quantum yields owing to the intercrossed excited state of local existed and charge transfer states. The PTA5 nanoparticles can be fabricated by encapsulation with a biocompatible polymer matrix. Upon excitation at 800 nm, these nanoparticles present a relatively large two-photon absorption cross section of 3.29 × 10⁶ GM. These nanoparticles also exhibit good photostability in water and thus can be utilized for bioimaging. The tissue-penetrating depths of up to 170 μm for hepatic vessels and 380 μm for blood vessels of mouse ear were achieved using PTA5 nanoparticles. Furthermore, PTA5 nanoparticles show impressive reactive oxygen species generation capability under the irradiation of a white light source. This can be attributed to the effective intersystem crossing between high-level excited state. Upon irradiation with white light (400-700 nm) at 50 mW cm^-2 for 5 min every other day, the tumor growth can be effectively suppressed in the presence of PTA5 nanoparticles. These findings demonstrate that PTA5 nanoparticles can be used as a photosensitizer for photodynamic therapy.

[12]aneN3-Based Gemini-Type Amphiphiles with Two-Photon Absorption Properties for Enhanced Nonviral Gene Delivery and Bioimaging

Although a plethora of nonviral gene vectors have been developed for potential gene therapy, imageable gemini surfactants with stimuli-responsiveness and high transfection efficiency are still scarce for gene delivery. Herein, three gemini amphiphiles (DEDPP-4/8/12) consisting of an aggregation-induced emission (AIE) central fluorophore: 5,6-diphenylpyrazine-2,3-diester (DEDPP), decorated with triazole-[12]aneN₃ as the hydrophilic moiety and alkyl chains of various lengths as the hydrophobic moiety, were designed and synthesized for trackable gene delivery via optical imaging. All three amphiphiles exhibited ultralow critical micelle concentrations (CMCs) (up to 3.40 × 10^-6 M), prominent two-photon absorption properties, and solvatochromic fluorescence. Gel electrophoresis assays demonstrated that the migration of plasmid DNA was completely retarded after condensation with these gemini amphiphiles at low concentrations (up to 10 μM). In addition, the ester bond in these amphiphiles may facilitate vector degradation and DNA release, in response to esterase and the acidic environment inside cells. Upon self-assembly with DOPE to form liposomes, DEDPP-8/DOPE achieved the best transfection efficiency in four cell lines, and the transfection efficiency of DEDPP-8/DOPE in HeLa cell lines was 23.5-fold higher than that of Lipo2000, which is unusually high for small organic molecule-based nonviral vectors. Furthermore, excellent transfection efficiency of DEDPP-8/DOPE was obtained in the presence of serum, and the red fluorescence protein (RFP) gene was successfully transfected in zebrafish embryos. Both one- and two-photon fluorescence imaging clearly demonstrated the delivery process of plasmid DNA. This study demonstrated that gemini-type amphiphiles composed of a two-photon fluorophore core conjugated with triazole-[12]aneN₃ via an ester bond afforded an unprecedentedly high transfection efficiency with excellent biocompatibility, which may provide new insights for the design and development of multifunctional nonviral gene vectors for imageable gene delivery.

Two-Photon Dual-Emissive Carbon Dot-Based Probe: Deep-Tissue Imaging and Ultrasensitive Sensing of Intracellular Ferric Ions

Carbon dots (CDs)-based nanoparticles have been extensively explored for biological applications in sensing and bioimaging. However, the major translational barriers to CDs for imaging and sensing applications include synthetic strategies to obtain monodisperse CDs with tunable structural, electronic, and optical properties in order to achieve high-resolution deep-tissue imaging, intracellular detection, and sensing of metal ions with high sensitivity down to nanomolar levels. Herein, we report a novel strategy to synthesize and develop a multifunctional nitrogen-doped CDs probe of different sizes using a new combination of carbon and nitrogen sources. Our results show that the structural characteristics (i.e., the surface density of emissive traps and bandgaps levels) depend on the size of the CDs, which ultimately influences their optical properties. This work also demonstrates the development of a two-photon dual-emissive fluorescent multifunctional probes (3-FCDs) by conjugating fluorescein isothiocyanate on the surface of nitrogen-doped CDs. 3-FCDs show excellent near-infrared two-photon excitation ability, single-wavelength excitation, high photostability, biocompatibility, low cytotoxicity, and good cell permeability. Using two-photon fluorescence imaging, our multifunctional probe shows excellent deep-tissue high-resolution imaging capabilities with penetration depth up to 3000 and 280 μm in hydrogel scaffold and pigskin tissue, respectively. The designed probe exhibits ultrasensitivity and specificity toward Fe³⁺ ions with a remarkable detection limit of 2.21 nM using two-photon excitation. In addition, we also demonstrate the use of multifunctional CDs probe for ultrasensitive exogenous and real-time endogenous sensing of Fe³⁺ ions and imaging in live fibroblasts with rapid response times for intracellular ferric ion detection.

Multiphoton microscopy: a personal historical review, with some future predictions

The historical development of multiphoton microscopy is described, starting with a review of two-photon absorption, and including two- and three-photon fluorescence microscopies, and second- and third-harmonic generation microscopies. The effects of pulse length on signal strength and breakdown are considered. Different contrast mechanisms, including use of nanoparticles, are discussed. Two new promising techniques that can be applied to multiphoton microscopy are described.

Hierarchical Nanostructuring of Porous Silicon with Electrochemical and Regenerative Electroless Etching

Hierarchically nanostructured silicon was produced by regenerative electroless etching (ReEtching) of Si powder made from pulverized anodized porous silicon. This material is characterized by ∼15 nm mesopores, into the walls of which tortuous 2-4 nm pores have been introduced. The walls are sufficiently narrow that they support quantum-confined crystallites that are photoluminescent. With suitable parameters, the ReEtching process also provides control over the emission color of the photoluminescence. Ball milling and hydrosilylation of this powder with undecylenic acid produces nanoparticles with hydrodynamic diameter of ∼220 nm that exhibit robust and bright luminescence that can be excited with either one ultraviolet/visible photon or two near-infrared photons. The long-lived, robust visible photoluminescence of these chemically passivated porous silicon nanoparticles is well-suited for bioimaging and theranostic applications.

Combination of [12]aneN3 and Triphenylamine-Benzylideneimidazolone as Nonviral Gene Vectors with Two-Photon and AIE Properties

Three nonviral gene vectors, TPA-BI-A/B/C, have been designed and synthesized by the combination of one or two hydrophilic [12]aneN₃ moieties and two-photon fluorescent triphenylamine-benzylideneimidazolone (TPA-BI) units through different ester linkage. Spectroscopic characterization demonstrated that TPA-BI-A/B/C had strong aggregation-induced emissions (AIE), large Stokes shifts (230, 284, and 263 nm), and large two-photon absorption cross sections (δ_2PA) (67, 592, and 80 GM). Gel electrophoresis indicated that the three compounds completely condensed DNA at 15 μM in the presence of DOPE, and showed the lipase- and pH-triggered reversible release of DNA and the fluorescent recognition of the different lengths of ssDNA and dsDNA. The optimal TPA-BI-C/DOPE-mediated luciferase and GFP activity was 146% and 290% higher than those of Lipo2000. The transfection process of DNA could be traced clearly through one- and two-photon fluorescence spectra, and displayed in a 3D-video. TPA-BI-C/DOPE successfully transfected the GFP gene into zebrafish, which was superior to Lipo2000 (192%). In conclusion, TPA-BI-C/DOPE is the first nonviral gene vector with the abilities of pH/lipase enzyme responsiveness, one/two-photon fluorescent tracking of intracellular delivery of DNA, and successful transfection in vivo and in vitro, even better than Lipo2000.

Demonstration of flat-top beam illumination in widefield multiphoton microscopy

Multiphoton microscopy provides a suitable technique for imaging biological tissues with submicrometer resolution. Usually a Gaussian beam (GB) is used for illumination, leading to a reduced power efficiency in the multiphoton response and vignetting for a square-shaped imaging area. A flat-top beam (FTB) provides a uniform spatial intensity distribution that equalizes the probability of a multiphoton effect across the imaging area. We employ a customized widefield multiphoton microscope to compare the performance of a square-shaped FTB illumination with that based on using a GB, for both two-photon fluorescence (TPF) and second-harmonic generation (SHG) imaging. The variation in signal-to-noise ratio across TPF images of fluorescent dyes spans ∼5.6  dB for the GB and ∼1.2  dB for the FTB illumination, respectively. For the GB modality, TPF images of mouse colon and Convallaria root, and SHG images of chicken tendon and human breast biopsy tissue showcase ∼20  %   area that are not imaged due to either insufficient or lack of illumination. For quantitative analysis that depends on the illuminated area, this effect can potentially lead to inaccuracies. This work emphasizes the applicability of FTB illumination to multiphoton applications.

Nonlinear optical spectroscopy and two-photon excited fluorescence spectroscopy reveal the excited states of fluorophores embedded in a beetle's elytra

Upon illumination by ultraviolet light, many animal species emit light through fluorescence processes arising from fluorophores embedded within their biological tissues. Fluorescence studies in living organisms are however relatively scarce and so far limited to the linear regime. Multiphoton excitation fluorescence analyses as well as nonlinear optical techniques offer unique possibilities to investigate the effects of the local environment on the excited states of fluorophores. Herein, these techniques are applied for the first time to study of the naturally controlled fluorescence in insects. The case of the male Hoplia coerulea beetle is investigated because the scales covering the beetle's elytra are known to possess an internal photonic structure with embedded fluorophores, which controls both the beetle's coloration and the fluorescence emission. An intense two-photon excitation fluorescence signal is observed, the intensity of which changes upon contact with water. A third-harmonic generation signal is also detected, the intensity of which depends on the light polarization state. The analysis of these nonlinear optical and fluorescent responses unveils the multi-excited states character of the fluorophore molecules embedded in the beetle's elytra. The role of form anisotropy in the photonic structure, which causes additional tailoring of the beetle's optical responses, is demonstrated by circularly polarized light and nonlinear optical measurements.

Tetraphenylethylene-Interweaving Conjugated Macrocycle Polymer Materials as Two-Photon Fluorescence Sensors for Metal Ions and Organic Molecules

A luminescent conjugated macrocycle polymer (CMP) with strong two-photon fluorescence property, namely, P[5]-TPE-CMP, is constructed from ditriflate-functionalized pillar[5]arene and a 1,1,2,2-tetrakis(4-ethynylphenyl)ethylene (TPE) linker through a Sonogashira-Hagihara cross-coupling reaction. Significantly, in sharp contrast with the corresponding conjugated microporous polymer without synthetic macrocycles, P[5]-TPE-CMP shows an outstanding stability against photobleaching and exhibits highly selective cation sensing capability toward Fe³⁺ at different excitation wavelengths (both UV and red-near-infrared regions). Meanwhile, its fluorescence could also be sufficiently quenched by 4-amino azobenzene, a frequently used organic dye that is certified to be carcinogenic, as compared with a group of common organic compounds. This work paves a new way for enhancing the properties of porous organic polymers through the introduction of supramolecular macrocycles like macrocyclic arenes.

Application of a reflective microscope objective for multiphoton microscopy

Reflective objectives (ROs) mitigate chromatic aberration across a broad wavelength range. Yet, a systematic performance characterisation of ROs has not been done. In this paper, we compare the performance of a 0.5 numerical-aperture (NA) reflective objective (RO) with a 0.55 NA standard glass objective (SO), using two-photon fluorescence (TPF) and second-harmonic generation (SHG). For experiments spanning ∼1 octave in the visible and NIR wavelengths, the SO leads to defocusing errors of 25-40% for TPF images of subdiffraction fluorescent beads and 10-12% for SHG images of collagen fibres. The corresponding error for the RO is ∼4% for both imaging modalities. This work emphasises the potential utility of ROs for multimodal multiphoton microscopy applications.

Midbrain-Hindbrain Boundary Morphogenesis: At the Intersection of Wnt and Fgf Signaling

A constriction in the neural tube at the junction of the midbrain and hindbrain is a conserved feature of vertebrate embryos. The constriction is a defining feature of the midbrain-hindbrain boundary (MHB), a signaling center that patterns the adjacent midbrain and rostral hindbrain and forms at the junction of two gene expression domains in the early neural plate: an anterior otx2/wnt1 positive domain and a posterior gbx/fgf8 positive domain. otx2 and gbx genes encode mutually repressive transcription factors that create a lineage restriction boundary at their expression interface. Wnt and Fgf genes form a mutually dependent feedback system that maintains their expression domains on the otx2 or gbx side of the boundary, respectively. Constriction morphogenesis occurs after these conserved gene expression domains are established and while their mutual interactions maintain their expression pattern; consequently, mutant studies in zebrafish have led to the suggestion that constriction morphogenesis should be considered a unique phase of MHB development. We analyzed MHB morphogenesis in fgf8 loss of function zebrafish embryos using a reporter driven by the conserved wnt1 enhancer to visualize anterior boundary cells. We found that fgf8 loss of function results in a re-activation of wnt1 reporter expression posterior to the boundary simultaneous with an inactivation of the wnt1 reporter in the anterior boundary cells, and that these events correlate with relaxation of the boundary constriction. In consideration of other results that correlate the boundary constriction with Wnt and Fgf expression, we propose that the maintenance of an active Wnt-Fgf feedback loop is a key factor in driving the morphogenesis of the MHB constriction.

Spectroscopic characterization of oral epithelial dysplasia and squamous cell carcinoma using multiphoton autofluorescence micro-spectroscopy

OBJECTIVE

Multiphoton autofluorescence microscopy (MPAM) has shown potential in identifying features that are directly related to tissue microstructural and biochemical changes throughout epithelial neoplasia. In this study, we evaluate the autofluorescence spectral characteristics of neoplastic epithelium in dysplasia and oral squamous cell carcinoma (OSCC) using multiphoton autofluorescence spectroscopy (MPAS) in an in vivo hamster model of oral neoplasia in order to identify unique signatures that could be used to delineate normal oral mucosa from neoplasia.

MATERIALS/METHODS

A 9,10-dimethyl-1,2-benzanthracene (DMBA) hamster model of oral precancer and OSCC was used for in vivo MPAM and MPAS. Multiphoton Imaging and spectroscopy were performed with 780 nm excitation while a bandpass emission 450-650 nm was used for MPAM. Autofluorescence spectra was collected in the spectral window of 400-650 nm.

RESULTS

MPAS with fluorescence excitation at 780 nm revealed an overall red shift of a primary blue-green peak (480-520 nm) that is attributed to NADH and FAD. In the case of oral squamous cell carcinoma (OSCC) and some high-grade dysplasia an additional prominent peak at 635 nm, attributed to PpIX was observed. The fluorescence intensity at 635 nm and an intensity ratio of the primary blue-green peak versus 635 nm peak, showed statistically significant difference between control and neoplastic tissue.

DISCUSSION

Neoplastic transformation in the epithelium is known to alter the intracellular homeostasis of important tissue metabolites such as NADH, FAD, and PpIX, which was observed by MPAS in their native environment. A combination of deep tissue microscopy owing to higher penetration depth of multiphoton excitation and depth resolved spectroscopy could prove to be invaluable in identification of cytologic as well as biomolecular spectral characteristic of oral epithelial neoplasia. Lasers Surg. Med. 49:866-873, 2017. © 2017 Wiley Periodicals, Inc.

Piezochromic Carbon Dots with Two-photon Fluorescence

Piezochromic materials, which show color changes resulting from mechanical grinding or external pressure, can be used as mechanosensors, indicators of mechano-history, security papers, optoelectronic devices, and data storage systems. A class of piezochromic materials with unprecedented two-photon absorptive and yellow emissive carbon dots (CDs) was developed for the first time. Applied pressure from 0-22.84 GPa caused a noticeable color change in the luminescence of yellow emissive CDs, shifting from yellow (557 nm) to blue-green (491 nm). Moreover, first-principles calculations support transformation of the sp² domains into sp³ -hybridized domains under high pressure. The structured CDs generated were captured by quenching the high-pressure phase to ambient conditions, thus greatly increasing the choice of materials available for a variety of applications.

Near-Infrared Photoluminescent Polymer-Carbon Nanodots with Two-Photon Fluorescence

Near-infrared-emissive polymer-carbon nanodots (PCNDs) are fabricated by a newly developed facile, high-output strategy. The PCNDs emit at a wavelength of 710 nm with a quantum yield of 26.28%, which is promising for deep biological imaging and luminescent devices. Moreover, the PCNDs possess two-photon fluorescence; in vivo bioimaging and red-light-emitting diodes based on these PCNDs are demonstrated.

Peptide-Decorated Tunable-Fluorescence Graphene Quantum Dots

We report here the synthesis of graphene quantum dots with tunable size, surface chemistry, and fluorescence properties. In the size regime 15-35 nm, these quantum dots maintain strong visible light fluorescence (mean quantum yield of 0.64) and a high two-photon absorption (TPA) cross section (6500 Göppert-Mayer units). Furthermore, through noncovalent tailoring of the chemistry of these quantum dots, we obtain water-stable quantum dots. For example, quantum dots with lysine groups bind strongly to DNA in solution and inhibit polymerase-based DNA strand synthesis. Finally, by virtue of their mesoscopic size, the quantum dots exhibit good cell permeability into living epithelial cells, but they do not enter the cell nucleus.

Precise Spatially Selective Photothermolysis Using Modulated Femtosecond Lasers and Real-time Multimodal Microscopy Monitoring

The successful application of lasers in the treatment of skin diseases and cosmetic surgery is largely based on the principle of conventional selective photothermolysis which relies strongly on the difference in the absorption between the therapeutic target and its surroundings. However, when the differentiation in absorption is not sufficient, collateral damage would occur due to indiscriminate and nonspecific tissue heating. To deal with such cases, we introduce a novel spatially selective photothermolysis method based on multiphoton absorption in which the radiant energy of a tightly focused near-infrared femtosecond laser beam can be directed spatially by aiming the laser focal point to the target of interest. We construct a multimodal optical microscope to perform and monitor the spatially selective photothermolysis. We demonstrate that precise alteration of the targeted tissue is achieved while leaving surrounding tissue intact by choosing appropriate femtosecond laser exposure with multimodal optical microscopy monitoring in real time.

Organic Dots Based on AIEgens for Two-Photon Fluorescence Bioimaging

Two-photon fluorescence imaging technique is a powerful bioanalytical approach in terms of high photostability, low photodamage, high spatiotemporal resolution. Recently, fluorescent organic dots comprised of organic emissive cores and a polymeric matrix are emerging as promising contrast reagents for two-photon fluorescence imaging, owing to their numerous merits of high and tunable fluorescence, good biocompatibility, strong photobleaching resistance, and multiple surface functionality. The emissive core is crucial for organic dots to get high brightness but many conventional chromophores often encounter a severe problem of fluorescence quenching when they form aggregates. To solve this problem, fluorogens featuring aggregation-induced emission (AIE) can fluoresce strongly in aggregates, and thus become ideal candidates for fluorescent organic dots. In addition, two-photon absorption property of the dots can be readily improved by just increase loading contents of AIE fluorogen (AIEgen). Hence, organic dots based on AIEgens have exhibited excellent performances in two-photon fluorescence in vitro cellular imaging, and in vivo vascular architecture visualization of mouse skin, muscle, brain and skull bone. In view of the rapid advances in this important research field, here, we highlight representative fluorescent organic dots with an emissive core of AIEgen aggregate, and discuss their great potential in bioimaging applications.

Organic Dots Based on AIEgens for Two-Photon Fluorescence Bioimaging

Two-photon fluorescence imaging technique is a powerful bioanalytical approach in terms of high photostability, low photodamage, high spatiotemporal resolution. Recently, fluorescent organic dots comprised of organic emissive cores and a polymeric matrix are emerging as promising contrast reagents for two-photon fluorescence imaging, owing to their numerous merits of high and tunable fluorescence, good biocompatibility, strong photobleaching resistance, and multiple surface functionality. The emissive core is crucial for organic dots to get high brightness but many conventional chromophores often encounter a severe problem of fluorescence quenching when they form aggregates. To solve this problem, fluorogens featuring aggregation-induced emission (AIE) can fluoresce strongly in aggregates, and thus become ideal candidates for fluorescent organic dots. In addition, two-photon absorption property of the dots can be readily improved by just increase loading contents of AIE fluorogen (AIEgen). Hence, organic dots based on AIEgens have exhibited excellent performances in two-photon fluorescence in vitro cellular imaging, and in vivo vascular architecture visualization of mouse skin, muscle, brain and skull bone. In view of the rapid advances in this important research field, here, we highlight representative fluorescent organic dots with an emissive core of AIEgen aggregate, and discuss their great potential in bioimaging applications.

Silole-Based Red Fluorescent Organic Dots for Bright Two-Photon Fluorescence In vitro Cell and In vivo Blood Vessel Imaging

Robust luminescent dyes with efficient two-photon fluorescence are highly desirable for biological imaging applications, but those suitable for organic dots fabrication are still rare because of aggregation-caused quenching. In this work, a red fluorescent silole, 2,5-bis[5-(dimesitylboranyl)thiophen-2-yl]-1-methyl-1,3,4-triphenylsilole ((MesB)2 DTTPS), is synthesized and characterized. (MesB)2 DTTPS exhibits enhanced fluorescence efficiency in nanoaggregates, indicative of aggregation-enhanced emission (AEE). The organic dots fabricated by encapsulating (MesB)2 DTTPS within lipid-PEG show red fluorescence peaking at 598 nm and a high fluorescence quantum yield of 32%. Upon excitation at 820 nm, the dots show a large two-photon absorption cross section of 3.43 × 10(5) GM, which yields a two-photon action cross section of 1.09 × 10(5) GM. These (MesB)2 DTTPS dots show good biocompatibility and are successfully applied to one-photon and two-photon fluorescence imaging of MCF-7 cells and two-photon in vivo visualization of the blood vascular of mouse muscle in a high-contrast and noninvasive manner. Moreover, the 3D blood vasculature located at the mouse ear skin with a depth of over 100 μm can also be visualized clearly, providing the spatiotemporal information about the whole blood vascular network.

Synthesis and Sensing Applications of Fluorescent 3-Cinnamoyl Coumarins

We have synthesized two novel fluorescent 3-(4-diethylaminocinnamoyl) coumarins that exhibit fluorescence quenching upon exposure to a nerve agent simulant, diethylchlorophosphate (DCP), providing a basis for rapid and sensitive DCP chemosensing. Furthermore, these coumarin derivatives display two-photon fluorescence upon illumination with near-infrared laser pulses and their two-photon (TP) absorption cross-section was evaluated. The potential for TP bio-imaging of these compounds was investigated by their cellular uptake in HeLa cells by TP confocal microscopy.

Carbon "Quantum" Dots for Fluorescence Labeling of Cells

The specifically synthesized and selected carbon dots of relatively high fluorescence quantum yields were evaluated in their fluorescence labeling of cells. For the cancer cell lines, the cellular uptake of the carbon dots was generally efficient, resulting in the labeling of the cells with bright fluorescence emissions for both one- and two-photon excitations from predominantly the cell membrane and cytoplasm. In the exploration on labeling the live stem cells, the cellular uptake of the carbon dots was relatively less efficient, though fluorescence emissions could still be adequately detected in the labeled cells, with the emissions again predominantly from the cell membrane and cytoplasm. This combined with the observed more efficient internalization of the same carbon dots by the fixed stem cells might suggest some significant selectivity of the stem cells toward surface functionalities of the carbon dots. The needs and possible strategies for more systematic and comparative studies on the fluorescence labeling of different cells, including especially live stem cells, by carbon dots as a new class of brightly fluorescent probes are discussed.

Effect of Size-Dependent Photodestructive Efficacy by Gold Nanomaterials with Multiphoton Laser

The photostability, photodestructive efficacy, two-photon excitation cross section, and two-photon fluorescence of gold nanoparticles conjugated with a hydrophilic photosensitizer, indocyanine green, via multiphoton laser exhibited an increased size effect in methicillin-resistant Staphylococcus aureus and A549 cancer cells that was dependent on the size of multifunctional gold nanomaterials, but the effect only occurred when nanomaterials within 100 nm in diameter were used. Besides, the enhanced effectiveness of photodestruction, photostability, and contrast probe indicated an additive effect in the therapeutic and imaging efficiency of multifunctional gold nanomaterials. Consequently, the preparation of the multifunctional gold nanomaterials and their use in biomedical applications via multiphoton laser is an alternative and potential therapeutic approach for killing bacteria and for ablating cancer cells.

Biocompatible Green and Red Fluorescent Organic Dots with Remarkably Large Two-Photon Action Cross Sections for Targeted Cellular Imaging and Real-Time Intravital Blood Vascular Visualization

Fluorescent organic dots are emerging as promising bioimaging reagents because of their high brightness, good photostability, excellent biocompatibility, and facile surface functionalization. Organic dots with large two-photon absorption (TPA) cross sections are highly desired for two-photon fluorescence microscopy. In this work, we report two biocompatible and photostable organic dots fabricated by encapsulating tetraphenylethene derivatives within DSPE-PEG matrix. The two organic dots show absorption maxima at 425 and 483 nm and emit green and red fluorescence at 560 and 645 nm, with high fluorescence quantum yields of 64% and 22%, respectively. Both organic dots exhibit excellent TPA property in the range of 800-960 nm, affording upon excitation at 820 nm remarkably large TPA cross sections of 1.2×10(6) and 2.5×10(6) GM on the basis of dot concentration. The bare fluorophores and their organic dots are biocompatible and have been used to stain living cells for one- and two-photon fluorescence bioimagings. The cRGD-modified organic dots can selectively target integrin αvβ3 overexpressing breast cancer cells for targeted imaging. The organic dots are also applied for real-time two-photon fluorescence in vivo visualization of the blood vasculature of mouse ear, providing the spatiotemporal information about the whole blood vascular network. These results demonstrate that the present fluorescent organic dots are promising candidates for living cell and tissue imaging.

Enhanced two-photon fluorescence imaging and therapy of cancer cells via Gold@bridged silsesquioxane nanoparticles

A two-photon photosensitizer with four triethoxysilyl groups is synthesized through the click reaction. This photosensitizer allows the design of bridged silsesquioxane (BS) nanoparticles through a sol-gel process; moreover, gold core BS shells or BS nanoparticles decorated with gold nanospheres are synthesized. An enhancement of the two-photon properties is noted with gold and the nanoparticles are efficient for two-photon imaging and two-photon photodynamic therapy of cancer cells.

Single-layered graphitic-C(3)N(4) quantum dots for two-photon fluorescence imaging of cellular nucleus

A promising, safe, and economic fluorescent probe, g-C3 N4 single-layered QDs, is introduced for two-photon fluorescence imaging of the cellular nucleus for the first time. The large two-photon absorption cross section, the high photostability, good biocompatibility and non-toxicity, negligible photothermal effect, and specific interaction with DNA render the single-layered g-C3 N4 QDs as a promising candidate for in vivo and in vitro two-photon fluorescence imaging and further biomedical applications.

Imaging directed photothermolysis through two-photon absorption demonstrated on mouse skin - a potential novel tool for highly targeted skin treatment

One-photon absorption based traditional laser treatment may not necessarily be selective at the microscopic level, thus could result in un-intended tissue damage. Our objective is to test whether two-photon absorption (TPA) could provide highly targeted tissue alteration of specific region of interest without damaging surrounding tissues. TPA based laser treatments (785 nm, 140 fs pulse width, 90 MHz) were performed on ex vivo mouse skin using different average power levels and irradiation times. Reflectance confocal microscopy (RCM) and combined second-harmonic-generation (SHG) and two-photon fluorescence (TPF) imaging channels were used to image before, during, and after each laser treatment. The skin was fixed, sectioned and H & E stained after each experiment for histological assessment of tissue alterations and for comparison with the non-invasive imaging assessments. Localized destruction of dermal fibers was observed without discernible epidermal damage on both RCM and SHG + TPF images for all the experiments. RCM and SHG + TPF images correlated well with conventional histological examination. This work demonstrated that TPA-based light treatment provides highly localized intradermal tissue alteration. With further studies on optimizing laser treatment parameters, this two-photon absorption photothermolysis method could potentially be applied in clinical dermatology.

Chemically specific imaging and in-situ chemical analysis of articular cartilage with stimulated Raman scattering

Stimulated Raman scattering (SRS) has been applied to unstained samples of articular cartilage enabling the investigation of living cells within fresh tissue. Hyperspectral SRS measurements over the CH vibrational region showed variations in protein and lipid content within the cells, pericellular matrix and interterritorial matrix. Changes in the cells and pericellular matrix were investigated as a function of depth into the cartilage. Lipid was detected in the pericellular matrix of superficial zone chondrocytes. The spectral profile of lipid droplets within the chondrocytes indicated that they contained predominantly unsaturated lipids. The mineral content has been imaged by using the PO₄³⁻ vibration at 959 cm⁻¹ and the CO₃²⁻ vibration at 1070 cm⁻¹. Both changes in cells and mineralization are known to be important factors in the progression of osteoarthritis. SRS enables these to be visualized in fresh unstained tissue and consequently should benefit osteoarthiritis research.

Ex vivo optical metabolic measurements from cultured tissue reflect in vivo tissue status

Optical measurements of metabolism are ideally acquired in vivo; however, intravital measurements are often impractical. Accurate ex vivo assessments would greatly broaden the applicability of optical measurements of metabolism. We investigate the use of live tissue culture experiments to serve as a surrogate for in vivo metabolic measurements. To validate this approach, NADH and FAD fluorescence intensity and lifetime images were acquired with a two-photon microscope from hamster cheek pouch epithelia in vivo, from biopsies maintained in live tissue culture up to 48 h, and from flash-frozen and thawed biopsies. We found that the optical redox ratio (fluorescence intensity of NADH/FAD) of the cultured biopsy was statistically identical to the in vivo measurement until 24 h, while the redox ratio of the frozen-thawed samples decreased by 15% (p<0.01). The NADH mean fluorescence lifetime (τm) remained unchanged (p>0.05) during the first 8 h of tissue culture, while the NADH τm of frozen-thawed samples increased by 13% (p<0.001). Cellular morphology did not significantly change between in vivo, cultured, and frozen-thawed tissues (p>0.05). All results were consistent across multiple depth layers in this stratified squamous epithelial tissue. Histological markers for proliferation and apoptosis also confirm the viability of tissues maintained in culture. This study suggests that short-term ex vivo tissue culture may be more appropriate than frozen-thawed tissue for optical metabolic and morphologic measurements that approximate in vivo status.

Increased illumination uniformity and reduced photodamage offered by the Lissajous scanning in fiber-optic two-photon endomicroscopy

We compare the illumination uniformity and the associated effects of the spiral and Lissajous scanning patterns that are commonly used in an endomicroscope. Theoretical analyses and numerical simulations were first performed to quantitatively investigate the area illumination density in the spiral scanning pattern. The results revealed the potential problem of manifest photodamage due to the very high illumination density in the center of the spiral scan. Similar analyses of the Lissajous scanning pattern, which can be conveniently implemented on the same endomicroscope with no hardware modifications, showed a more uniform illumination density with about an 80-fold reduction in the peak illumination density. To underscore the benefit offered by the improved illumination uniformity, we conducted in vitro two-photon fluorescence imaging of cultured cells stained with a LIVE/DEAD viability assay using our home-built, fiber-optic, two-channel endomicroscopy system. Both the spiral and the Lissajous scans were implemented. Our experimental results showed that cells near the spiral scan center experienced obvious photodamage, whereas cells remained alive over the entire region under the Lissajous beam scanning, confirming the predicted advantage offered by the Lissajous scan over this spiral scan in an endomicroscopy setting.

Two-photon fluorescence diagnostics of femtosecond laser tweezers

We show how two-photon fluorescence signal can be used as an effective detection scheme for trapping particles of any size in comparison to methods using back-scattered light. Development of such a diagnostic scheme allows us a direct observation of trapping a single nanoparticle, which shows new directions to spectroscopy at the single-molecule level in solution.

Selective two-photon fluorescence suppression by ultrafast pulse-pair excitation: control by selective one-color stimulated emission

Controlling two-photon molecular fluorescence leading to selective fluorophore excitation has been a long sought after goal in fluorescence microscopy. In this letter, we thoroughly explore selective fluorescence suppression through simultaneous two-photon absorption by two different fluorophores followed by selective one-photon stimulated emission for one particular fluorophore. We achieve this by precisely controlling the time delay between two identical ultrafast near infrared laser pulses.

Fiber-optic multiphoton flow cytometry in whole blood and in vivo

Circulating tumor cells in the bloodstream are sensitive indicators for metastasis and disease prognosis. Circulating cells have usually been monitored via extraction from blood, and more recently in vivo using free-space optics; however, long-term intravital monitoring of rare circulating cells remains a major challenge. We demonstrate the application of a two-photon-fluorescence optical fiber probe for the detection of cells in whole blood and in vivo. A double-clad fiber was used to enhance the detection sensitivity. Two-channel detection was employed to enable simultaneous measurement of multiple fluorescent markers. Because the fiber probe circumvents scattering and absorption from whole blood, the detected signal strength from fluorescent cells was found to be similar in phosphate-buffered saline (PBS) and in whole blood. The detection efficiency of cells labeled with the membrane-binding dye 1,1'-dioctadecyl-3,3,3',3'-tetramethylindoldicarbocyanine, 4-chlorobenzenesulfonate (DiD) was demonstrated to be the same in PBS and in whole blood. A high detection efficiency of green fluorescent protein (GFP)-expressing cells in whole blood was also demonstrated. To characterize in vivo detection, DiD-labeled untransfected and GFP-transfected cells were injected into live mice, and the cell circulation dynamics was monitored in real time. The detection efficiency of GFP-expressing cells in vivo was consistent with that observed ex vivo in whole blood.

Towards Spatio-Temporal Control in Optical Trapping

Using both continuous-wave (CW) and high repetition rate femtosecond lasers, we present stable 3-dimensional trapping of 1μm polystyrene microspheres. We also stably trapped 100nm latex nanoparticles using the femtosecond mode-locked laser at a very low average power where the CW lasers cannot trap, demonstrating the significance of the fleeting temporal existence of the femtosecond pulses. Trapping was visualized through dark-field microscopy as well as through a noise free detection using two-photon fluorescence as a diagnostics tool owing to its intrinsic 3-dimensional resolution. Comparison between a Gaussian versus a flat-top Gaussian beam profile demonstrates the importance of laser spatial mode in optical trapping.

Non-Linear Optical Imaging of Obesity-Related Health Risks: Review

This review highlights the recent applications of non-linear optical (NLO) microscopy to study obesity-related health risks. A strong emphasis is given to the applications of coherent anti-Stokes Raman scattering (CARS) microscopy where multiple non-linear optical imaging modalities including CARS, sum-frequency generation (SFG), and two-photon fluorescence are employed simultaneously on a single microscope platform. Specific examples on applications of NLO microscopy to study lipid-droplet biology, obesity-cancer relationship, atherosclerosis, and lipid-rich biological structures are discussed.

Neural transplant staining with DiI and vital imaging by 2-photon laser-scanning microscopy

We are developing a multielectrode silicon "neuroprobe" for maintaining a long-term, specific, two-way electrical interface with nervous tissue. Our approach involves trapping a neuron (from an embryonic rat hippocampus) in a small well with a stimulation/recording electrode at its base. The well is covered with a grillwork through which the neuron's processes are allowed to grow, making synaptic contact with the host tissue, in our case a cultured slice from a rat hippocampus. Each neuroprobe can accommodate 15 neurons, one per well. As a first step in studying neurite outgrowth from the neuroprobe, it was necessary to develop new staining techniques so that neurites from the probe neurons can be distinguished from those belonging to the host, without interference from non-specific background staining. We virtually eliminated background staining through a number of innovations involving dye solubility, cell washing, and debris removal. We also reduced photobleaching and phototoxicity, and enhanced imaging depth by using a 2-photon laser-scanning microscope. We focused on using the popular membrane dye, DiI, however a number of other membrane dyes were shown to provide clear images of neural processes using pulsed illumination at 900 nm. These techniques will be useful to others wishing to follow over time the growth of neurons in culture of after transplantation in vivo, in a non-destructive way.