Two-photon Excited Fluorescence Enhancement Research Articles

Tip-enhanced two-photon excited fluorescence spectroscopy of monolayer black phosphorus

The optical properties of monolayer black phosphorus (BP) in two-photon excited fluorescence (TPEF) are pivotal for photoluminescence applications at the single molecular level. In this study, we theoretically examine the TPEF properties of monolayer BP using a tip-enhanced spectroscopy system. We detail the influence of parameters such as the tip radius, the distance between the tip and the BP film, the incidence angle and the tip half-cone angle on the fluorescence excitation and emission enhancement of monolayer BP. This examination employs the finite element method. Our findings reveal that the signal enhancement in TPEF is substantially improved, approximately two orders of magnitude greater than that in single-photon excited fluorescence (SPEF), attributed to the high-order nonlinear coupling effect. By optimising the system configuration, we determined that the TPEF enhancement factor with monolayer BP can reach as high as eight orders of magnitude under an excitation wavelength of 800 nm. The highest spatial resolutions achievable for TPEF and SPEF spectroscopy are also 4.4 nm and 5.3 nm, respectively. These results provide an in-depth understanding of the TPEF enhancement mechanism and are invaluable for designing and applying BP-based optoelectronic devices.

Enhanced two-photon excited fluorescence by ultrafast intensity fluctuations from an optical parametric generator

We evaluate the enhancement of two-photon excited fluorescence from fluorescent dye by ultrafast intensity fluctuations of light generated by a nanosecond optical parametric generator (OPG). By modifying the well-established techniques for measuring the second-order coherence on ultrashort timescale, we observed the enhancement of the fluorescence from Rhodamine B dye by a factor of two due to the bunching effect of light. A nanosecond OPG can be built at a much lower cost than the ultrashort pulse used in two-photon excitation microscopes. Therefore, our work paves the way toward a low-cost and efficient nonlinear microscope.

Efficient two-photon fluorescent probe with red emission for imaging of thiophenols in living cells and tissues.

Thiophenols, a class of highly toxic and pollutant compounds, are widely used in industrial production. Some aliphatic thiols play important roles in living organisms. Therefore, the development of efficient methods to discriminate thiophenols from aliphatic thiols is of great importance. Although several one-photon fluorescent probes have been reported for thiophenols, two-photon fluorescent probes are more favorable for biological imaging due to its low background fluorescence, deep penetration depth, and so on. In this work, a two-photon fluorescent probe for thiophenols, termed NpRb1, has been developed for the first time by employing 2,4-dinitrobenzene-sulfonate (DNBS) as a recognition unit (also a fluorescence quencher) and a naphthalene-BODIPY-based through-bond energy transfer (TBET) cassette as a fluorescent reporter. The TBET system consists of a D-π-A structured two-photon naphthalene fluorophore and a red-emitting BODIPY. It displayed highly energy transfer efficiency (93.5%), large pseudo-Stokes shifts upon one-photon excitation, and red fluorescence emission (λem = 586 nm), which is highly desirable for bioimaging applications. The probe exhibited a 163-fold thiophenol-triggered two-photon excited fluorescence enhancement at 586 nm. It showed a high selectivity and excellent sensitivity to thiophenols, with a detection limit of 4.9 nM. Moreover, it was successfully applied for practical detection of thiophenol in water samples with a good recovery, two-photon imaging of thiophenol in living cells, and tissues with tissue-imaging depths of 90-220 μm, demonstrating its practical application in environmental samples and biological systems.

激发光的表面等离激元增强效应导致的双光子荧光增强

Two-photon-excited fluorescence (TPEF) enhancement effect in CdSe quantum dot modified Au nanoparticle was theoretically investigated. TPEF enhancement factor was obtained in dipole approximation by taking surface plasmon resonance enhancement effect and nonradiative energy transfer to Au NP into account. An available method to achieve sufficient TPEF enhancement factor with noble metal nanoparticles was concluded from the analysis of numerical simulation. Greater TPEF enhancement effect will be obtained by adjusting the surface plasmon resonant peak of noble metal nanoparticles to the excitation wavelength. This theoretical analysis result was proved to be appropriate to the reported experimental results.

Two-photon excited fluorescence enhancement with broadband versus tunable femtosecond laser pulse excitation

The inverse relationship between two-photon excited fluorescence (TPEF) and laser pulse duration suggests that two-photon microscopy (TPM) performance may be improved by decreasing pulse duration. However, for ultrashort pulses of sub-10 femtosecond (fs) in duration, its spectrum contains the effective gain bandwidth of Ti:Sapphire and its central wavelength is no longer tunable. An experimental study was performed to explore this apparent tradeoff between untuned sub-10 fs transform-limited pulse (TLP) and tunable 140 fs pulse for TPEF. Enhancement factors of 1.6, 6.7, and 5.2 are measured for Indo-1, FITC, and TRITC excited by sub-10 fs TLP compared with 140 fs pulse tuned to the two-photon excitation (TPE) maxima at 730 nm, 800 nm, and 840 nm, respectively. Both degenerate (v(1) = v(2)) and nondegenerate (v(1) ≠ v(2)) mixing of sub-10 fs TLP spectral components result in its broad second-harmonic (SH) power spectrum and high spectral density, which can effectively compensate for the lack of central wavelength tuning and lead to large overlap with dye TPE spectra for TPEF enhancements. These pulse properties were also exploited for demonstrating its potential applications in multicolor imaging with TPM.

A small molecule two-photon probe for hydrogen sulfide in live tissues

We report a two-photon probe (FS1) which shows a 21-fold two-photon excited fluorescence enhancement in response to H(2)S and can selectively detect H(2)S in a rat hippocampal slice at a depth of 90-190 μm by using two-photon microscopy.

A highly sensitive two-photon fluorescent probe for mitochondrial zinc ions in living tissue

We report a highly sensitive two-photon probe (SZn2-Mito) which shows a 70-fold two-photon excited fluorescence enhancement in response to Zn(2+) and can selectively detect mitochondrial Zn(2+) in a rat hippocampal slice at a depth of 100-200 μm by using two-photon microscopy.

Enhancement of two-photon excited fluorescence in two-dimensional nonlinear optical polymer photonic crystal waveguides

We examined two-photon excited fluorescence (TPEF) in two-dimensional highly nonlinear optical polymer photonic crystal waveguides with metallic cladding. A large TPEF enhancement of approximately two orders of magnitude was observed, originating from resonance between the external pump laser field and a photonic band mode. The results for TPEF enhancement are in good agreement with experimental photonic band resonances obtained by angle-resolved reflectivity measurements and the theoretical photonic band structure generated by three-dimensional finite-difference time-domain calculations, indicating that it is possible to use band dispersion control to design artificial nonlinear two-photon excitation processes.

Enhancement of two-photon excited fluorescence using one-dimensional photonic crystals

We fabricated a one-dimensional photonic crystal structure with a defect layer of a poly(vinyl alcohol) thin film doped with 2-aminopurine (2AP). The defect induced a transmission peak in the photonic band gap at 610 nm, to which ultrashort laser pulses were tuned. We observed enhanced two-photon fluorescence emission from 2AP in the photonic crystal structure with a factor of 120. The enhancement was attributed to the high local field of light generated by a photonic state localized at the defect layer. Furthermore, under the enhanced light intensity, we carried out photobleaching experiments, which gave useful information on the photochemistry of 2AP.