Fine Regulation of Porous Architectures of Core-Shell Silica Nanocomposites Offers Robust Nanoprobes with Accelerated Responsiveness

The design strategies and biological applications of probes for the gaseous signaling molecule hydrogen sulfide

H₂S, the smallest and simplest biological thiol in living systems, is the third member of the family of signaling mediators. H₂S participates in the regulation of a series of complex physiological and pathological functions in the body, making it a critical fulcrum that balances health and disease in human physiology. Small-molecule fluorescent probes have been proven to possess the unique advantages of high temporal and spatial resolution, good biocompatibility and high sensitivity, and thus their use is a powerful approach for monitoring the level and dynamics of H₂S in living cells and organisms and better understanding its basic cellular functions. The field of small-molecule fluorescent probes for monitoring the complex biological activities of H₂S in vivo has been thriving in recent years. Herein, we systematically summarize the latest developments in the field of fluorescent probes for the detection of H₂S, illustrate their biological applications according to the classification of target-responsive sites, and emphasize the development direction and challenges of H₂S-responsive fluorescent probes, hoping to give implications of researchers on fluorescent probes for future research.

A phase-transfer catalyst-based nanoreactor for accelerated hydrogen sulfide bio-imaging

Hydrogen sulfide (H₂S) is an important signaling molecule in various biological processes; however, its real-time monitoring in living cells is hampered by long detection time for current fluorescent probes. To overcome this challenge, we designed a phase-transfer catalyst (PTC) approach to accelerate the reaction between the probe and the analyte by conjugating common fluorescent probes - mostly hydrophobic small molecules - with an amphiphilic PEG-PPG-PEG polymer, enabling the controllable assembly of H₂S nanoprobes in an aqueous solution. The PEG block helps to establish a PTC microenvironment that endows the assembled nanoprobes with a significantly reduced detection time (3-10 min; versus 20-60 min for small-molecule probes). Based on this approach, we synthesised two nanoprobes of different wavelengths, DS-Blue-nano and DN-Green-nano, which can sensitively detect H₂S in living macrophage cells with bright fluorescence starting at as early as 7 min and reaching stability at 15 min. These data suggest PTC-based nanoprobes as a new and generic approach for constructing sensitive fluorescent probes for the real-time imaging of H₂S, and perhaps other molecules in future, under biological conditions.

An Activatable Afterglow/MRI Bimodal Nanoprobe with Fast Response to H2 S for In Vivo Imaging of Acute Hepatitis

Accurate detection of hepatic hydrogen sulfide (H₂ S) to monitor H₂ S-related enzymes' activity is critical for acute hepatitis diagnosis, but remains a challenge due to the dynamic and transient nature of H₂ S. Here, we report a H₂ S-activatable near-infrared afterglow/MRI bimodal probe F1-GdNP, which shows an "always-on" MRI signal and "off-on" afterglow signal toward H₂ S. F1-GdNP shows fast response, high sensitivity and specificity toward H₂ S, permitting afterglow imaging of H₂ S and evaluation of cystathionine γ-lyase (CSE)'s activity in living mice. We further employ the high spatial-resolution MRI signal of F1-GdNP to track its delivery and accumulation in liver. Importantly, F1-GdNP offers a high signal-to-background ratio (SBR=86.2±12.0) to sensitively report on the increased hepatic H₂ S level in the acute hepatitis mice via afterglow imaging, which correlated well with the upregulated CSE activity in the liver, showcasing the good potential of F1-GdNP for monitoring of acute hepatitis process in vivo.

Aggregation Enhanced Responsiveness of Rationally Designed Probes to Hydrogen Sulfide for Targeted Cancer Imaging

Activatable molecular probes hold great promise for targeted cancer imaging. However, the hydrophobic nature of most conventional probes makes them generate precipitated agglomerate in aqueous media, thereby annihilating their responsiveness to analytes and precluding their practical applications for bioimaging. This study reports the development of two small molecular probes with unprecedented aggregation enhanced responsiveness to H₂S for in vivo imaging of H₂S-rich cancers. The subtle modulation of the equilibrium between hydrophilicity and lipophilicity by N-methylpyridinium endows these designed probes with the capability of spontaneously self-assembling into nanoprobes under physiological conditions. Such probes in an aggregated state, rather than a molecular dissolved state, show NIR fluorescence light up and photoacoustic signals turn on upon H₂S specific activation, allowing in vivo visualization and differentiation of cancers based on differences in H₂S content. Thus, our study presents an effective design strategy which should pave the way to molecular design of optimized probes for precision cancer diagnostics.

Rational design of water-dispersible and biocompatible nanoprobes with H2S-triggered NIR emission for cancer cell imaging

We present an approach for constructing a H₂S-specific nanoprobe by the entrapment of a small molecule probe within the hydrophobic interior of surface cross-linked micelles (SCMs), endowing the designed nanoprobes with good water solubility and biocompatibility. Importantly, the obtained nanoprobes displayed good responsiveness to H₂S in both ratiometric fluorescence and light-up NIR emission modes, thus enabling accurate identification of H₂S-rich colorectal cancer cells.

Curcumin-Cu(II) Ensemble-Based Fluorescence "Turn-On" Mode Sensing the Plant Defensive Hormone Salicylic Acid In Situ and In Vivo

Salicylic acid (SA), a crucial, plant-derived signal molecule, is capable of launching global transcriptional reprogramming to assist plants in obtaining the systemic acquired resistance (SAR) mechanism. Thus, the accurate detection of SA will not only significantly contribute to the understanding of the plant SAR but also contribute to crop protection and to the security of the agricultural production and food supply. However, detection of SA using fluorescent probes is a great challenge for scientists, because SA analogues can significantly interfere with the detection results. Herein, we first reported using a simple, natural curcumin-Cu²⁺ ensemble to selectively and sensitively monitor SA in situ and in vivo, directed by a fluorescence "turn-on" mode. A binary combination curcumin-Cu²⁺ was first fabricated with a fluorescence "turn-off" pattern caused by the paramagnetic nature of Cu²⁺. Subsequently, a fluorescence "turn-on" response was performed for detecting SA accompanied by the formation of the ternary complex curcumin-Cu²⁺-SA due to the high affinity of SA toward Cu²⁺, which reduced the fluorescent impact caused by the paramagnetism of Cu²⁺. Further study revealed that the rationally designed hybrid probe could monitor SA in living cell lines. We anticipate that this finding can inspire the discovery of a high-performance SA probe.

An electron-deficiency-based framework for NIR-II fluorescence probes

Increasing the electron withdrawing ability of substituents in monochlorinated BODIPY could vary the emission from the NIR-I to NIR-II region together with enhanced response rate, indicative of a promising approach for activatable NIR-II probes.

Activatable near-infrared emission-guided on-demand administration of photodynamic anticancer therapy with a theranostic nanoprobe

Development of theranostic probes that can be used to identify tumors and direct the on-demand drug administration to cancers is ongoing but remains challenging. Herein, we report a theranostic platform composed of a H2S-activated imaging probe and a light-sensitive drug. The designed probe affords advantages of H2S-activated NIR emission light-up and efficient 1O2 generation, enabling the selective visualization of H2S-rich cancers and the subsequent imaging-directed on-demand light exposure to the detected cancers while leaving normal tissues untouched. Such controllable administration of photodynamic anticancer therapy maximizes the therapeutic efficiency and minimizes side effects. This work should facilitate significant advances toward precise diagnosis and treatment of cancer.

Mitochondria-targeting BODIPY-loaded micelles as novel class of photosensitizer for photodynamic therapy

In this paper, a series of novel BODIPY-based photosensitizers have been designed and synthesized for photodynamic therapy. BODIPY3 was screened out as the most potential photosensitizer due to its excellent optical properties, high singlet oxygen efficiency and good photostability. However, as an organic photosensitizer, BODIPY3 still suffered from the drawbacks of insolubility and instability in aqueous system. In view of these problems, DSPE-PEG2000 was used to trap BODIPY3 into the hydrophobic core of micelles to obtain well-dispersing nano complexes BODIPY3-PEG₃ in aqueous system. More importantly, BODIPY3-PEG₃ not only has better solubility and stability in aqueous media but can generate significant singlet oxygen (¹O₂, one of the reactive oxygen species, the real cytotoxic agent in photodynamic therapy) in living cells and exhibit high light cytotoxicity to three cancer cell lines. The mechanism studies indicated the mitochondrial localization of BODIPY3-PEG₃ was able to generate ROS in mitochondria, which further result in mitochondrial dysfunction and photoinduced apoptosis via caspase-8 and caspase-3 pathway.

Selective tracking of ovarian-cancer-specific γ-glutamyltranspeptidase using a ratiometric two-photon fluorescent probe

Real-time tracking of GGT enzymatic activity in human ovarian cancer cells is a reliable method for accurate prediction of cancer diagnosis and management.