Smart H2S‐Triggered/Therapeutic System (SHTS)‐Based Nanomedicine

Abstract Hydrogen sulfide (H2S) is of vital importance in several biological and physical processes. The significance of H2S‐specific detection and monitoring is emphasized by its elevated levels in various diseases such as cancer. Nanotechnology enhances the performance of chemical sensing nanoprobes due to the enhanced efficiency and sensitivity. Recently, extensive research efforts have been dedicated to developing novel smart H2S‐triggered/therapeutic system (SHTS) nanoplatforms for H2S‐activated sensing, imaging, and therapy. Herein, the latest SHTS‐based nanomaterials are summarized and discussed in detail. In addition, therapeutic strategies mediated by endogenous H2S as a trigger or exogenous H2S delivery are also included. A comprehensive understanding of the current status of SHTS‐based strategies will greatly facilitate innovation in this field. Lastly, the challenges and key issues related to the design and development of SHTS‐based nanomaterials (e.g., morphology, surface modification, therapeutic strategies, appropriate application, and selection of nanomaterials) are outlined.


Introduction
Hydrogen sulfide (H 2 S) is a highly toxic gas known for its causticity, flammability and distinct odor of rotten eggs. [1][2][3] However, endogenous H 2 S is the third major gasotransmitter in addition to carbon monoxide (CO) and nitric oxide (NO). [4][5][6] The misregulation of this signaling molecule is associated with numerous diseases, such as Alzheimer's disease, diabetes, and cancer. [7,8] Since H 2 S has such a crucial role, an effective H 2 S detection method would facilitate the understanding of the implicated diseases permit early diagnosis. Currently, the most-used techniques include high-pressure liquid/gas chromatography (HPLC/GC) have shown significant sensitivity. However, the high cost and Hydrogen sulfide (H 2 S) is of vital importance in several biological and physical processes. The significance of H 2 S-specific detection and monitoring is emphasized by its elevated levels in various diseases such as cancer. Nanotechnology enhances the performance of chemical sensing nanoprobes due to the enhanced efficiency and sensitivity. Recently, extensive research efforts have been dedicated to developing novel smart H 2 S-triggered/ therapeutic system (SHTS) nanoplatforms for H 2 S-activated sensing, imaging, and therapy. Herein, the latest SHTS-based nanomaterials are summarized and discussed in detail. In addition, therapeutic strategies mediated by endogenous H 2 S as a trigger or exogenous H 2 S delivery are also included. A comprehensive understanding of the current status of SHTS-based strategies will greatly facilitate innovation in this field. Lastly, the challenges and key issues related to the design and development of SHTS-based nanomaterials (e.g., morphology, surface modification, therapeutic strategies, appropriate application, and selection of nanomaterials) are outlined.
As such a promising field, smart H 2 S-triggered/therapeutic system (SHTS)-based nanomedicine is expected to significantly accelerate the development of disease diagnosis and therapeutic strategy by enhancing accuracy and efficiency. Given the vital role of hydrogen sulfide in biological processes and advantages of nanotechnology, we provide an overview of recent progress in H 2 S detection, imaging and related disease therapy via SHTS-based nanomedicine (Figure 1). Within this review, various nanoagents such as noble metal nanomaterials, metal-organic framework, copper-based nanomaterials, and carbon nanodot for H 2 S sensing, different imaging (including fluorescence, localized surface plasmon resonance, upconversion luminescence, near-infrared, photoacoustic and positron emission tomography imaging) and therapeutic strategies (e.g., the endogenous H 2 S-triggered therapy or exogenous H 2 S delivery) are summarized. As such, we aim to highlight these powerful nanoprobes in this emerging field and offer an overview for the development of next-generation of SHTS-based nanomedicine.

Roles of H 2 S in Biological Systems
Endogenous H 2 S is mainly produced from cysteine by three enzymes: 3-mercaptopyruvate sulfotransferase (3-MST), cystathionine β-synthase (CBS), and cystathionine γ-lyase (CSE). [58][59][60][61] The H 2 S generated is a vital gas transmitter that affects various biological and physical functions within the body, ranging from antiinflammation to regulation of neuronal transmission. [62][63][64][65] For instance, it has been reported that H 2 S donors promote the production of ATP and electron transport in mitochondrial. [66] Furthermore, H 2 S is able to protect the cell by attenuating apoptosis.
Thus, it has been widely applied as a novel reagent for preserving organs from ischemia-reperfusion injury during various surgeries and organ transplantations. [67][68][69] Also, the increased secretion of endogenously H 2 S is strongly associated with the progress of tumor. [4,70] Notably, the H 2 S generating enzymic system including 3-MST, CBS, and CSE have been widely identified in many cancer types. [4,71] The overexpression of CBS has been particularly reported within various colon and ovarian cancers, [72,73] indicating the significant role of H 2 S in promoting tumor development. The hydrogen sulfide derived from cancer cells also promotes tumor growth and proliferation by acting as an autocrine and paracrine factor. [72] After introducing a CBS inhibitor, the growth of colon cancer could be greatly attenuated by efficiently reducing H 2 S generation and inhibiting peritumor angiogenesis. [61] However, the fast catabolism and regulation of this toxic gas show a great challenge for real-time detection within the tissues. [74] As one of the most dangerous gases, the concentration of H 2 S within the air needs to be monitored as well. While this toxic gas easily noted because of its rotten-egg smell, the exposure to H 2 S can cause a serial of symptoms including lung irritations (≤20 ppm), damage of eye (300-500 ppm), unconsciousness, or even death (≥700 ppm). [75] Therefore, successful detection/imaging of hydrogen sulfide would be immensely valuable for disease diagnosis and treatment, as well as risk management.

H 2 S Detection with SHTS-Based Nanomedicine
To monitor H 2 S in solutions and air, various nanomaterials have been developed as novel sensors, including noble metal nanoparticles (e.g., Au, Ag, and Au/Ag alloy), metal-organic frameworks (MOF), copper nanomaterials, carbon nanodots, among others (e.g., ruthenium nanoparticles, etc.). In this section, a series of SHTS-based nanosensors will be summarized ( Table 2).  is a well-known dye for glass staining that can be traced back to the Roman era. With the excellent stability, catalytic ability, and optical properties, gold, silver, and alloy nanomaterials have been widely developed and applied for biomedical engineering applications. [43,85,87] The LSPR is a key characteristic of noble metal nanomaterial that is easily influenced by the size, Adv. Sci. 2019, 6,1901724  distance, and composition. [104] Based on this property, a variety of detection methods have been developed by the formation/ dispersion of aggregation or change of the surface, including the specific detection for hydrogen sulfide (Table 2). With proper surface functionalization using different ligands such as glutathione, [77] fluorosurfactants, [82] or small molecules (e.g., thiolated azido derivates and active esters), [44] gold nanosensors quickly aggregate when they encounter with H 2 S. This results in a redshift of absorbance wavelengths and LOD ranging from 0.2 × 10 −6 to 20 × 10 −6 m. In comparison, hydrophobic surface modification (e.g., fluorescent probe, 1-(10-mercaptodecyl)-5-methylpyrimidine-2,4-dione, TSH) force the AuNDs coated with TSH and MUA (11-mercaptoundecanoic acid) to aggregate. The presence of H 2 S could disassemble the aggregation surface adsorption of H 2 S and HS, recovering the quantum yield back to 1.61%. [79] Similarly, Zhang et al. developed a simple sensing strategy by using bubbling H 2 S to stabilize the AuNPs (13 nm), with the existence of NaCl (80 × 10 −3 m) and Tween 80 (Figure 2A). [84] This cost-effective method provides a high sensitivity toward H 2 S with LOD values reaching around 14 × 10 −6 m for the naked eye and 30 × 10 −9 m for machine detection, which is more efficient than that afforded by TSH-MUA-AuNDs (0.5 × 10 −6 m).
While other approaches, such as the change of LSPR induced by surface reduction and competitive binding between S-Au and I-Au (forming clusters or larger nanoparticles), [80,81] have been used with gold-based sensors, the sensing limits only reach about 0.3 × 10 −6 m for H 2 S detection. Comparably, catalysis mediated by Au based nanosensors has excellent sensitivity. [83,85] A catalysis Au@TPt-NCs (Au core with an ultrathin platinum shell) nanoplatform was developed by Gao et al. to detect dissolved H 2 S gas ( Figure 2B). [85] The H 2 S evaporated or dissolved interacts with and deactivates the nanoclusters, attenuating the chromogenic reaction between H 2 O 2 and 3,3′,5,5′-tetramethylbenzidine (TMB) and showing an extremely low LOD value at 7.5 × 10 −9 m. More importantly, the approach is also visible to the naked, providing flexibility for applications ( Figure 2C,D).

Metal-Organic Framework (MOF)
The past decade has seen drawn a great deal of attention to metal-organic framework (MOF) due to their excellent physiochemical features. [105][106][107][108][109][110] These nanomaterials are composed of different combinations of metal ions, organic linkers, and modifications and have vast application possibilities (e.g., gas storage, chemical sensing, chiral separations, etc.). [111] With the tremendous surface area (≈7000 m 2 g −1 ) and rigid pores that could host various functional molecules, MOF has also been investigated as a potential sensor for chemical and toxic gas detection, such as hydrosulfide. [112] Through the formation of the metal sulfides (e.g., CuS), [46,89,90] amine group, [39] or NS bond [91] with S 2− , several novel MOFs could recover the fluorescence/luminescence that was quenched and trigger a detectable signal for sensing H 2 S with a desirable sensitivity. For instance, the presence of Tb 3+ /Cu 2+ ions enables the Tb 3+ @Cu1/Cu2 MOF complex to generate multiwavelength luminescence and produce an enhanced ratiometric signal (I 544 /I 390 ) after the interaction with the H 2 S exposed, with a LOD of S 2− at about 1.2 × 10 −6 m. [89] Similarly, Qian Lab synthesized an Eu 3+ @UiO-66-(COOH) 2 MOF that induced a fluorescent signal via the interaction between Cu 2+ and S 2− . [46] Although such MOF exhibits a uniform nanostructure (80-100 nm) and comparable H 2 S LOD (5.45 × 10 −6 m), the fluorescence intensity generated could be affected by amino acids containing thiol and nitroxyl groups, which strongly lowers selectivity toward H 2 S. [46] Comparably, the novel sensors, Zr(TBAPy) 5 (TCPP) and aluminum-based MOF (Al-MIL-53-NO 2 ) demonstrate desirable H 2 S detection and selectivity via reduction, with LOD of ≈92.31 × 10 −9 m and ≈1 ppb, respectively. [39,91] The Zr(TBAPy) 5 (TCPP) were synthesized with a uniform nanostructure (with a diameter around 100 nm) after incorporation of Tris(2-chloroisopropyl)phosphate (TCPP) (Figure 3A,B). [91] This synthesized nanoparticle was very sensitive to H 2 S (with a LOD around 50 × 10 −9 m), and only showed fluorescence after the introduction of H 2 S, demonstrating a desirable linear relation between fluorescence and the concentration of H 2 S ( Figure 3C,D,F). More importantly, the reaction of Zr(TBAPy) 5 (TCPP) and H 2 S was completed within 10 s, providing an opportunity for real-time detection ( Figure 3E).
As alternatives to single substrate MOFs, probes for the detection of multiple biomolecules are highly desirable for large scale detection in environmental or clinical assay. Recently, a Eu 3+ -Cu 2+ based MOF was developed. [90] With two specific and separate binding areas for ascorbic acid (AA) and H 2 S, it simultaneously detected both biomolecules. Due to the high sensitivities, the as-prepared MOF can identify H 2 S and AA concentrations as lower as 130 × 10 −9 and 55 × 10 −9 m, respectively. Additionally, desirable recovery rate (94.7-104.1%) was attained in assays using human serum. After incorporating various elements and molecules, novel MOF-based probes for multiple biomolecule detection have significant promise for biomedical applications.

Copper Based Nanomaterials
Copper (Cu), the most-used cation for H 2 S sensing (via the metal precipitation), has been widely incorporated into small organic molecules (e.g., HSIP-1) and the other nanosized probes. [78,80] The addition of Cu to nanomaterials in the form of Cu, CuO, or Cu 2 O is also employed for H 2 S-specific detection. [52] Coating the surface of nanoparticle sensors (e.g., nanowires, nanoneedles, or nanotubes) with Cu, CuO, or Cu 2 O enables rapid detection of H 2 S due to variations in conductivity after reduction. As such, the concentration of H 2 S in the solution or air can be determined. [53,92,93] For example, a Cu 2 O NPs (2-3 nm) coated WO 3 nanoneedles were prepared via aerosol-assisted chemical vapor deposition (AACVD). [53] This system was able to detect H 2 S levels as low as 300 ppb within two seconds. A major limitation of the Cu 2 O-WO 3 nanoneedles was the high temperature required (390 °C) that makes practical application difficult. A Quasi-2D-Cu 2 O/SnO 2 consisting of P-type Cu 2 O and N-type SnO 2 was successfully developed for H 2 S gas detection at room temperature, with a LOD at 0.5 ppm. [94] Notably, laser illumination further reduced the heterojunction barrier and enhanced the response of Quasi-2D-Cu 2 O/SnO 2 by 20%. The Chen lab synthesized a Cu 2 O-FGS (functional graphene sheets) by in situ growth that provided desirable surface accessibility, contacting area (Cu 2 O was prepared without surfactant) and sensitivity (LOD is around 5 ppb) for H 2 S gas sensing under normal atmospheric conditions ( Figure 4A-C). [52] Several studies have confirmed that the aggregation of organic Cu NCs (e.g., cysteine or penicillamine (PAE) template) can activate an enhanced fluorescence referred to as aggregation-induced emission (AIE). [95,96] By incorporating polystyrene sulfonate (PSS) into the system, PSS-PAE-Cu NCs aggregates were designed H 2 S detection in drinking water ( Figure 4D). [95] With the 0.05 wt% PSS, as-prepared PSS-PAE-Cu NCs aggregates (173 nm) generated red photoluminescence (665 nm) that was extinguished when as little as 650 × 10 −9 m H 2 S was present ( Figure 4E,F).

Carbon Nanodot
Since the first discovery at 2004, carbon nanodots (C-dots or CDs) have been widely investigated for biomedical, catalytic, and sensing applications due to its attractive features of high solubility, biocompatibility, and photostability. [113][114][115] Among all, several novel C-dots have been designed for H 2 S detection/imaging. [51,97,116,117] For example, two metal ion (Ag + /Hg + ) based C-dots were synthesized for sensing sulfide ions. In the presence of H 2 S as low as 0.32 × 10 −6 and 0.43 × 10 −6 m respectively, the fluorescence of CD-Hg + /Ag + would be quenched by the inner filter effect (IFE) mediated by the Hg 2 S/Ag 2 S formed. Meanwhile, the formation of Ag 2 S significantly changes the Ag-C-dot's electrochemiluminescence that shows a desirable sensitivity with a LOD at 27 × 10 −9 m. [56,57] Adv. Sci. 2019, 6,1901724

H 2 S Imaging with SHTS-Based Nanomedicine
As we have mentioned, micro-or nano-probes have been widely applied for H 2 S measurements in clinical samples and has greatly facilitated bench efforts. However, the real-time imaging of H 2 S secretion in patients for disease diagnosis, especially tumor tracking, is still highly demanded. Among all the in vitro and in vivo imaging candidates, nanocarriers have shown great potential as fluorescent, LSPR, upconversion luminescence (UCL), near infrared (NIR), photoacoustic (PA) and positron emission tomography (PET) imaging probes ( Table 3).
In comparison with the fluorescence and UCL imaging, only a few of nanoprobes has been used for H 2 S imaging via NIR, LSRP, PA, and PET. These nanosensors would be described in detail in this section.

Fluorescence Imaging
With the innovation of imaging technology, two-and multiphoton microscopy has been used for fluorescence imaging, which greatly improved the depth of penetration (≈1 mm). [118,119] However, most fluorescent agents have generally employed for living cell or tissue section based imaging. For instance, the incorporation of organic components (azide or unsaturated CC bond) enabled MOF to be used for specific imaging in a series of cancer cell lines (e.g., PC12 and J774A.1 cells) via reduction mediated by H 2 S. [120,121] Comparably, Adv. Sci. 2019, 6,1901724  . Reproduced with permission. [44] Copyright 2017, American Chemical Society.
further functionalization with Cu 2+ based ligands (e.g., Cyclam-Cu 2+ ) enabled the C-Dots to visualize H 2 S within cells via fluorescence initiated after CuS precipitation. [40,116,117,122] These nanosensors demonstrated desirable biocompatibility and efficiently detected H 2 S in Hela or L929 cells, with LOD ranging from around 90 × 10 −9 -780 × 10 −9 m. Notably, C-Dot-TPEA-Cu 2+ , a two-photon nanoprobe, exhibited excellent tumor penetration that could be used for sensing H 2 S in A549 tumor sections. This system provided an emission wavelength (560 nm) suitable to minimize background for H 2 S and nuclei imaging, compared with those (≈460 nm) offered by other C-dots. [116] Internal Förster resonance energy transfer (FRET) is able to aid specific imaging of H 2 S in vivo. The FRET acceptor (e.g., CuO coated on the surface) or probe structure changes could initiate or terminate FRET in response to H 2 S. [45,49,50] Notably, carried fluorophores can change its excitation or emission wavelength to act as the imaging trigger when exposed to H 2 S. [49] For instance, boron-dipyrromethene (BODIPY), with a small Stokes shift and high fluorescent quantum yields, has been widely employed in various nanosized platforms for probing H 2 S. A micellar nanomaterial was designed by incorporating an amphiphilic copolymer (mPEGDSPE), semi-cyanine-BODIPY hybrid dye (BODInDCI), and BODIPY1 as the energy donor for H 2 S imaging. (Figure 6A). [49] Once the BODInDCl was exposed to H 2 S, its absorption wavelength rapidly shifted from 540 to 738 nm and suspend FRET between it and BODIPY1, eventually recovering and switching off fluorescence at 511 and 589 nm ( Figure 6B). Importantly, this reaction was quickly Adv. Sci. 2019, 6,1901724  finished within 140 s, demonstrating high efficiency for H 2 S detection. [129] Additionally, this nanoBODIPY probe was able to track endogenous H 2 S in a macrophage cell line (RAW 264.7) based on the ratio between the dual-color images ( Figure 6C,D). [49]

LSPR Dark-Field Imaging
Plasmonic nanoparticles (PNPs), such as gold nanorods (AuNR), can provide extremely bright signal compared with organic fluorescent dyes. [130,131] By further coating Ag on the AuNR, Xiong et al. successfully applied the gold nanorodsilver (AuNR-Ag) core-shell PNPs for mapping H 2 S n living cells via dark-field imaging. [41] The AuNR-Ag PNP (74 × 19 nm core and 2.1 nm shell) generated Ag 2 S and changed its LSPR wavelength when it encountered with H 2 S (Figure 7A,B).
of H 2 S sensors had responsive emission signals ranging from the visible to the NIR region. [34] These UCNPs-chromophores showed various LRET efficiency (11.8-25.1%), but all exhibited high excellent selectivity and rapid responsiveness in live cells and blood serum. Doping Tm 3+ into UCNPs introduces UCL signals at 800 nm that can be utilized as an internal standard for ratiometric detection of H 2 S to improve sensitivity. As an example, Liu et al. employed NaYF 4 :20%Yb,2%Er,0.2%Tm@ mSiO 2 -merocyanines for ratiometric detection of H 2 S using multiwavelength UCL. [151] UCNPs@mSiO 2 -MC showed an enhanced ratiometric signal (I 540 /I 800 ) for higher sensitivity with LOD at ≈0.58 × 10 −6 m, which was lower than that of another merocyanine-based H 2 S probe (1.0 × 10 −6 m). [152] Similarly, Zhou et al. used NaYF 4 :20%Yb,1.8%Er,0.5%Tm@αcyclodextrin (CD)-coumarin hemicyanine (CHC1) dye as a ratiometric UCL probe (Figure 8A). [153] By measuring the ratio of I 580 /I 800, this UCNPs was able to measure H 2 S concentrations as low as 0.13 µm, much more sensitive than single UCL signals (1.85 µm) in aqueous solution ( Figure 8B). This UCNPs@ CD-CHC1 could be used for ratiometric UCL monitoring of pseudo-enzymatic H 2 S production in living cells, and also showed for ability to detect lipopolysaccharide (LPS)-induced inflammation in the liver tissues of mouse models for the first time ( Figure 8C).
UCNPs have also been developed for detecting or imaging of small molecules, biomacromolecules, organs, and tumors. Li et al. developed a merocyanine derivative modified UCNPs (NaYF 4 : 20%Yb, 2%Er, 0.2%Tm)@PEG as a ratiometric UCL probe for H 2 S detection in mitochondria of live cells and livetissues (Figure 9A-C). [154] This probe was used for locating the HCT116 (human colorectal cancer cell line) tumor in vivo by using NIR UCL imaging ( Figure 9D-F). Additionally, this system was capable of monitoring mitochondrial H 2 S within tumor slices via a ratiometric UCL measurement ( Figure 9G).
To monitor H 2 S using UCL imaging both ex vivo and in vivo, Wang et al. proposed a PAA-UCNPs (NaYF 4 :Yb/Tm@NaYF 4 ) loaded with a cyanine chromophore (Cy7-Cl) as a NIR probe for H 2 S response. (Figure 10A). [155] This nanoprobe was able to emit luminescence at 800 nm ( Figure 10B,C) and demonstrated superb sensitivity toward H 2 S ( Figure 10D,E). In addition to imaging exogenous and endogenous H 2 S in living cells (Hela and MCF-7 cells), the Cy7-UCNPs were successfully employed for sensing H 2 S in tumor-bearing zebrafish in real time, with high penetration depth and low autofluorescence background ( Figure 10F,G). Thus, the UCNPs-chromophores were capable of monitoring H 2 S in living cells and small animals by UCL imaging. Ratiometric UCL-based nanosystems provide a new design strategy for sensing and imaging of H 2 S that might be further utilized by novel probes for highly sensitive in vivo imaging studies.

NIR Imaging
Various fluorescent probes have been successfully employed for detection of cellular H 2 S. However, most of these fluorescent probes emit in the ultraviolet or visible light region (450-750 nm that is impeded by cell autofluorescence. In contrast, long wavelength probes with emission in the NIR region are optimal for biological imaging applications due to minimal photodamage to biological samples and interference from background autofluorescence in living systems. [156,157] Additionally, NIR light (700-900 nm) can well improve the     (n = 3). G) The two single channels and ratiometric UCL images of the tumor sections from the mice intravenously administrated with TPAMC-UCNPs@PEG and further injected with PBS (top raw) and S-adenosyl-l-methionine (SAM) (button raw). The green (500-560 nm) and red (600-680 nm) signal of UCL were obtained under a 980 nm excitation. Reproduced with permission. [156] Copyright 2018, American Chemical Society. tissue depth penetration for in vivo imaging. [158,159] Among NIR fluorochromes, cyanine dyes have excellent photophysical properties, such as outstanding biocompatibility and low toxicity to living systems, which is suitable incorporation as a fluorescent probe. [159,160] For example, Wang et al. designed a NIR fluorescent cyanine probe Cy-NO 2 (em. ≈789 nm) for H 2 S detection (via nitro group reduction) in aqueous solution and living cells. [161] Similarly, Zhang et al. reported a cyaninebased NIR probe (em. ≈796 nm) for a highly sensitive (with LOD at 39.6 × 10 −9 m) and selective imaging of endogenous H 2 S in tissues and tumor models (HCT116 and HT29) of mice. [162] Recent progress has demonstrated that fluorescence imaging in the second near-infrared window (NIR-II, 1000-1700 nm) can further improve image contrast at increased tissue depths. Moreover, NIR-II fluorescence imaging remarkably reduces interference from photon absorption and displays higher in vivo spatial resolution than NIR-I imaging. [163] Zhao's group fabricated an H 2 S-triggered NIR-II nanoprobe for visualizing colorectal cancers (Figure 11A). [164] The nanoprobes were comprised of a silica shield and two organic chromophores, a borondipyrromethene dye generating the NIR-II emission (em. 900-1300 nm) with the presence of H 2 S and an inert aza-BODIPY dye (em. 700 nm) as the internal reference ( Figure 11B,C). The NIR-II@Si showed a selective identification of H 2 S rich colon cancer cells via a dual color imaging modality (Figure 11D,F,G). Moreover, NIR-II@Si was further explored for the H 2 S-triggered NIR-II imaging with the supporting of SAM (S-adenosyll-methionine, the CBS activator) (HCT-116 tumors), showing enhanced deep tissue penetration and spatial resolution ( Figure 11E). [164]

PA Imaging
Among imaging methods that are not fluorescence-based, PA imaging is a newly emerging technique. This modality is based on the PA effect of translation of excitation light into ultrasonic waves, which bridges the traditional depth and resolution limits of conventional optical imaging techniques. [165,166] As the acoustic waves are generated by pulsed laser light, noninvasive biomedical images with sharp optical absorption contrast and high ultrasonic resolution are produced. [167,168] The development of chemical PA probes proposed a new perspective for monitoring therapeutic response and real-time molecular imaging. [169,170] For H 2 S detection, Shi et al. first presented a PA probe by encapsulating semi-cyanine-BODIPY hybrid dyes into the core-shell silica nanocomposites (Si@BODPA), enabling real-time imaging of H 2 S-related biological processes ( Figure 12A). [171] Based on the thiol-halogen nucleophilic substitution reaction, the Si@BODPA produced emission at 780 nm after the hydrogen sulfide activation, leading to a 44-fold turn-on response within 15 s ( Figure 12B,C). The LOD was determined to be as low as 53 × 10 −9 m, a sufficient sensitivity for detecting endogenous H 2 S within living systems. Due to its rapid response, Si@BODPA was then employed for the real-time monitoring of endogenous H 2 S generation in HCT116 tumor-bearing mouse to verify elevated level of H 2 S due to CBS upregulation ( Figure 12D).
Ratiometric PA probes are able to further eliminate some of the shortcomings of a single responsive PA signal by self-calibration. Thus, the combination of two PA responsive signals at two separated wavelengths would efficiently improve the accuracy of results.  C) The UV-vis absorption spectra of Cy7-Cl (black) along with Cy7-Cl + Na 2 S (blue), and the UCNPs' luminescence spectrum (red). D) The change of luminescence spectra upon the addition of various Na 2 S concentrations (0 × 10 −6 -100 × 10 −6 m). E) The enhancement of fluorescence ratio accompanied by increasing concentrations of Na 2 S. F) In vivo UCL images of exogenous and endogenous H 2 S in zebrafish via the Cy7-UCNPs imaging system: a,b) normal zebrafish were injected with PBS, followed by an administration of Cy7-UCNPs 30 min later; c,d) tumor-bearing zebrafish was administrated with PBS, followed by an injection of Cy7-UCNPs 30 min later; e,f) tumor-bearing zebrafish was first injected with NMM (the scavenger of intracellular H 2 S), followed by an injection of Cy7-UCNPs 30 min later; g,h) tumor-bearing zebrafish was administrated with l-Cys (the precursor of H 2 S), followed by the administration of Cy7-UCNPs 30 min later; the length of scale bar is 500 µm. G) The corresponding average UCL intensities of data in (a,c,e,g). Reproduced with permission. [157] Copyright 2018, Elsevier. photoacoustic nanoprobe AzHD (H 2 S-responsive NIR dye) that was carried by a liposome for monitoring and imaging of H 2 S in cells, brain tissues, and live mice. [127] With H 2 S-mediated reduction of the azide, the AzHD-LP absorption centered at 600 nm gradually decreased, and a new absorption band at 700 nm subsequently appeared (Figure 13A). Through this design, the ratio of PA700/PA532 increased about 4.5-fold after reactive with H 2 S, which was about 23-fold higher than a single Adv. Sci. 2019, 6,1901724   PA signal alone ( Figure 13B,C). The LOD of ratiometric PA signals was determined to be 91 × 10 −9 m. This enabled the ratio of PA700/PA532 PA signal of healthy and Alzheimer's disease (AD) mice brains (homogenate supernatant) to increase by 6.5 and 1.2-fold, respectively, following AzHD-LP introduction ( Figure 13D,E). Additionally, further conjugating the RGD targeting group to the AzHD-LP allowed for successful monitoring of H 2 S in the HCT116 tumor-bearing mice using timedependent dual-channel ratiometric PA signals ( Figure 13F-I). Therefore, the newly designed ratiometric PA probes of H 2 S sensing system provides a powerful analytical and imaging tool for further exploration of the roles of H 2 S in living complex organisms.

PET Imaging
Although fluorescence-based imaging techniques are primarily utilized for H 2 S detection, their applications in live-animal imaging are limited because of the limited quantitative analysis. PET provides a highly sensitive non-invasive technology for molecular imaging assays of metabolism, signal transduction, and gene expression from mice to patients. [172][173][174] Unsurprisingly, targeted and sensitive PET probes have also been developed for H 2 S imaging. As an example, Yoo's group utilized 64 CuS nanoparticles for the detection, quantification, and in vivo imaging of endogenous H 2 S via PET imaging. [128] These nanoparticles were formed by twenty macrocyclic 64 Cu complexes reacting with gaseous H 2 S to form insoluble 64 CuS (Figure 14A). 64 Cu-cyclen showed high sensitivity (with a LOD at 0.15 µm) and selectivity for H 2 S over other potential competitors, including polysulfides. Due to the physical differences, the intravenously injected 64 Cucyclen and 64 Cu-cyclam were quickly cleared from the body, while the insoluble 64 CuS nanoparticles were immobilized for more than 4 h after encountering H 2 S ( Figure 14B). When 64 Cucyclen was administrated into mice intravenously, an elevated H 2 S concentration within the inflamed paw was visualized and quantified by both PET imaging and Cerenkov luminescence ( Figure 14C,D). Moreover, the 64 Cu-cyclen could be also used to detect the defect site in the myocardium from an acute myocardial infarction (MI) model ( Figure 14E-H). As such, this radioactive probe demonstrated great potential as a powerful nanoplatform providing efficient detection, accurate quantification, and nuclear imaging of H 2 S within living animals.

SHTS-Based Nanomedicine for Disease Therapy
Following disease diagnosis, an effective, timely, and in situ treatment is highly demanded. In comparison to imaging agents, smart nanoplatforms could combine imaging, diagnosis, and therapy simultaneously. As highly-expressed H 2 S within the disease area as a trigger, multifunctional Adv. Sci. 2019, 6, 1901724   Figure 13. A) Schematic of the ratiometric photoacoustic AzHD-LP system, the change of the AzHD chemical structures, and PA absorbance (decrease of 532 nm and enhancement of 700 nm) after H 2 S exposure. B) The variation of two PA intensity (532 and 700 nm) of AzHD-LP under different NaSH concentrations. C) The enhancement of PA700/PA532 ratio with increasing concentration of NaSH. D) The optical images of brain tissue from the normal mice and the mice with Alzheimer's Disease (AD). E) The plot of the PA700/PA532 ratio obtained from the AzHD-LP after incubation with the brain homogenates from normal or AD mice. F) Schematic of the formation of RGD-AzHD-LP. G) The overlayed imaging of PA (PA700 and PA532) or ratiometric PA (PA700/PA532) with ultrasound acquired from the mice bearing subcutaneous HCT116 tumor. H) The corresponding quantitative intensity plot of PA532 (green) and PA700 (red) in (G). I) The ratiometric intensity of PA700/PA532 obtained from different groups at four hours post the intravenous administration of RGD-AzHD-LP: a) 12-hour preinjection of PBS in the tumor area; b) 12-hour preinjection of SAM (300 nmol) in the tumor area; c) 12-hour preinjection of AOAA (100 nmol) in the tumor area; d) 12-hour preinjection of ZnCl 2 (H 2 S trapper) in the tumor area; the length of scale bar is 5 mm. Reproduced with permission. [129] Copyright 2018, Royal Society of Chemistry.
nanoagents can serve as imaging and therapeutic agents simultaneously. As mentioned previously, the H 2 S functions as an important biological indicator and also has vital roles in a series of physiological functions, such as factors for protecting or killing cells. However, the application of most H 2 S donors is restricted by the short half-life and low hydrophilic property. Due to these limitations, several H 2 S-releasing nanomaterials were developed for various disease therapies. In this following section, these latest nanoagents designed for tumor diagnosis and treatment enabled by endogenous H 2 S activation will be discussed ( Table 4). Additionally, the exogenous H 2 S delivering nanoplatforms employed for tumor therapy, ischemic/reperfusion protection, and transplanted organ preservation will be summarized.

Endogenous H 2 S-Triggered Photodynamic Therapy
Under a specific wavelength (e.g., near-infrared light), photosensitizing agents generate reactive oxygen species (ROS) for treatment of diseases such as bacterial infection or cancers, referred to as photodynamic therapy (PDT). [178][179][180] Compared with conventional therapies such as chemotherapy and radiotherapy, PDT is an ideal strategy to treat cancer (i.e., lead the cellular apoptosis and necrosis via the ROS activated) since it is noninvasive, safe, and convenient. [181] However, photosensitizing agents (e.g., porphyrin) typically cannot elicit an antitumor PDT effect due to their physiochemical features (e.g., hydrophobic) nor are able to diagnose cancer. As such, nanomaterial alternatives have arisen as an attempt to effectively implement this therapeutic strategy. As an example, Ma et al. developed a smart, H 2 S-triggered MOF nanosensor acted as a photosensitizer after exposure to H 2 S (Figure 15). [55] This novel MOF, (Cu 2 (ZnTcpp)·H 2 O) n (NP-1) was synthesized using a reverse microemulsion system followed by a hydrothermal treatment. NP-1 reacted quickly with H 2 S within one minute to recover red fluorescence (≈Em610 and Em660). A linear logarithmic relationship was found for the fluorescence intensity and NaHS concentration (from 10-70 × 10 −6 m) (Figure 15A,B). As a potential photosensitizer, the NP-1 showed better PDT efficacy than the ZnTCPP precursor ( Figure 15C). Specifically, NP-1 (10 × 10 −6 m) responded only to laser irradiation (600 nm) to generate Adv. Sci. 2019, 6,1901724  . Reproduced with permission. [130] Copyright 2016, Wiley.
1 O 2 when H 2 S (50 × 10 −6 m) was present. In comparison, when not irradiated or H 2 S was absent, NP-1 was unable to damage to HepG2 human liver cancer cells ( Figure 15C). After intratumoral injection and irradiation, NP-1 was detrimental to the HCT-116 cells (high H 2 S levels) and nearly eradicated the entire tumor ( Figure 15D-G). Tumor shrinkage was also observed for mice injected with ZnTcpp following irradiation, but the therapeutic effect was relatively poor compared with NP-1. The role of H 2 S in irradiation-induced damage was confirmed using HCT-116 cells ( Figure 15H). Although this intelligent nanoplatform, NP-1 shows significant potential as a H 2 S-selective photosensitizing agent for PDT of cancer, further functionalization using PEGylation to enable the whole body circulation is highly recommended.

Endogenous H 2 S-Triggered Photothermal Therapy
As an additional photodynamic treatment, photothermal therapy (PTT) can damage or kill cancer cells by generating vibrational energy in the form of heat after electromagnetic radiation. [37,[182][183][184][185] Many nanomaterials, including gold nanorods and graphene, have been employed as PTT photosensitizers using NIR excitation. [157,186] Nevertheless, the scatted nanoagent would cause further damage to surrounding normal tissues after a laser applied. Therefore, targeting or selective ability is strongly required. Recently, an innovative nanoagent (Nano-PT) was synthesized via self-assembly of a H 2 S activated small molecule that is consist of a hydrophilic tail and a BODIPY core (Figure 16A,B). [175] As previously mentioned, the absorption wavelength of BODIPY changed after interaction with H 2 S. The variation (i.e., the change of wavelength from Ab540 to Ab790) enables the Nano-PT to absorb NIR irradiation (785 nm laser, 5.37 W cm −2 ) and produce heat that can reach around 55° after 10 min of irradiation ( Figure 16C). However, the temperature of the Nano-PT solution only slightly increased when H 2 S was absent. After the introduction of H 2 S, a bright NIR-II fluorescence signal (around Em950) was activated, and continuingly enhanced in a time-dependent pattern, with a LOD value at 106 × 10 −9 m ( Figure 16D). With such effective sensitivity, the HCT-116 tumor could be identified from normal tissue 2 h post the subcutaneous injection of Nano-PT ( Figure 16E). Importantly, a 20-degree temperature difference between normal (41.8 °C) and tumor (60.9 °C) tissue could well prevent accidental injury of nearby tissue. Furthermore, PTT mediated by Nano-PT successfully ablated the HCT-116 tumor and limited any noticeable damage to the surrounding healthy tissue (Figure 16F,G).
By intratumorally or subcutaneously administration, these nanoplatforms are able to treat noticeable tumors with PDT and PTT. However, these strategies are limited for clinical applications that often require simultaneous diagnosis and therapy. To achieve this, Yang's lab recently designed a H 2 S activated nanomaterial, Cu 2 O (21 nm), for colon cancer (HCT-116, CBS overexpression) theranostics ( Figure 17A). [54] After encountering endogenous H 2 S at the tumor site, Cu 2 O formed Cu 9 S 8 which absorbed NIR irradiation (808 nm) and increased the tumor tissue temperature by 20.7 °C. Additionally, the formation of Cu 9 S 8 provided a stable PA imaging agent that was unaffected by pH variations or GSH. For better efficiency, SAM (S-adenosyl-l-methionine) or AOAA (aminooxyacetic acid) were administered by intravenous injection as a CBS activator and inhibitor, respectively. After supplementation of Cu 2 O with SAM, increased PA intensity was found at the tumor site ( Figure 17B,C). While PA signal from Cu 2 O was detected, it failed to identify the tumor area due to its relatively lower intensity. Similarly, the CBS activator dramatically enhanced the temperature elevation with SAM + Cu 2 O treatment (15 °C), which was twice that of the Cu 2 O treated mice ( Figure 17D,E). After two weeks of treatment with SAM + Cu 2 O and laser irradiation, HCT-116 tumor-bearing mice were completely eradicated ( Figure 17F-H). In comparison, the size of the tumor treated with Cu 2 O + irradiation only slightly decreased. Thus, the reported Cu 2 O nanoparticle was an intelligent theranostic agent for clinic application after supplementation with SAM.

Nanoplatforms as Exogenous H 2 S Delivery System
Low concentrations of H 2 S are widely known to aid the proliferation of cancer cells and surrounding vessels. [4,61] However, sufficient H 2 S quickly released in tumor tissue affects cellular metabolism and has a toxic effect on tumor cells. [4] Exploiting this, Liu et al designed a H 2 S-generating "nanobomber" for cancer therapy (Figure 18A). [56] This nanoliposomes (AML) was loaded with the H 2 S donors, anethole dithiolethione (ADT) and magnetic nanoparticles (MNPs), and had a diameter around 200 nm. The ADT could be activated enzymatically to continuingly release significant H 2 S gas, eventually Adv. Sci. 2019, 6,1901724  forming microsized bubbles ( Figure 18B). The H 2 S bubbles rapidly occupied most of the intracellular space and caused the apparent morphology changes,) which was strongly cytotoxic to HepG2 cells, with more than 40% death after 12 h ( Figure 18C). These microbubbles were detected using ultrasonic imaging. After loading with MNPs, the AML accumulated in the tumor area under a magnetic field, which was around 3.4 times of that of Als (without MNPs) at 4 h postinjection ( Figure 18D-F). Ultrasonic treatment was then applied was applied to burst the intratumoral microsized bubbles and subsequently induce physical damage and H 2 S-induced cytotoxicity to the tumor tissue. The magnetic-guided as therapy successfully induced cell apoptosis (with 21.5 ± 7.4%) and suppressed the tumor growth up to 7 days. However, treatment without the magnetic field showed relatively lower therapeutic effect and decreased apoptosis rates (15.4 ± 4.5%) ( Figure 18G). In conclusion, this combined imaging system strongly enhanced the targeting accuracy during the treatment w will also providing the "H 2 S air bomber" for a novel cancer therapy strategy. Supplementation of H 2 S can help preserve organs and protect injuries triggered by ischemia/reperfusion by various antiapoptotic, antiinflammatory and antioxidative methods. [187][188][189] However, most H 2 S donors cannot produce decent protection due to burst release and poor solubility, such as the NaHS or diallyl sulfide (DATS). Mesoporous silica nanoparticles (MSNs) have arisen as ideal nanoplatforms due to their large surface Adv. Sci. 2019, 6,1901724  area that can be diversely functionalized, adjustable pore size for loading various cargo (e.g., the hydrophobic drug), and overall biocompatibility. [190,191] Recently, Wang's lab successfully developed DATS-loaded MSNs as a H 2 S-generating platform for protecting organs from I/R injury and transplantation. [57,176,177] These MSN (175-225 nm) efficiently carried DATS at the surface pore (≈2 nm) because of the high affinity between DATS and Si-OH, with an entrapment rate around 99% (Figure 19A). [176] A sustained DATS release profile (reaching about 80 min) was achieved after loaded on MSN and in the presence of GSH in the solution. In turn, the amount of H 2 S released from DATS alone quickly declined after only one hour. The supplementation of DATS-MSN in the preserving solution effectively reduced inflammation in the transplanted organ by downregulating the expression level of intercellular adhesion molecule-1 (ICAM-1) and vascular cell adhesion molecule-1 (VACM-1). [177] Notably, the DATS-MSN continuingly released H 2 S into the plasma for up to 12 h, while NaHS and DATS quickly decreased after one or three hours respectively ( Figure 19B). [57] The administration of DATS-MSN reduced myocardial apoptosis by approximately 15% at 24 h post-reperfusion. Additionally, DATS-MSN and substantially decreased I/R injury in myocardial tissues, which was confirmed using TTC staining (percentage of infarction area (INF)/area at risk (AAR)) ( Figure 19C,D). More importantly, the DATS-MSN exhibited superior protection of the heart after I/R injury in comparison to GYY4137, a conventional H 2 S donor with slow release kinetics ( Figure 19E,F).
Adv. Sci. 2019, 6, 1901724   Figure 16. A) Schematic of Nano-PT synthesis, the chemical structures of the components, and the transformation of SSS after the presence of H 2 S. B) Schematic illustration of the NIR-II-guided photothermal therapy for colorectal cancer mediated by Nano-PT nanoplatform. C) The temperature curves of PBS, Nano-PT, and Nano-PT + NaHS (100 × 10 −6 m) under laser irradiation. D) The change of NIR-II fluorescence spectra of Nano-PT during a series of time points (0-15 min) with NaHS (100 × 10 −6 m), and the NIR-II image of Nano-PT after the H 2 S activation. E) The NIR-II in vivo images of the normal and HCT-116 tumor tissue on nude mice after on-site subcutaneous injection of Nano-PT at different time points. F) The ratios of tumor weight (W d15 /W d0 ) among tumors collected from different groups (1) Control; 2) Nano-PT; 3) Laser; 4) Nano-PT + Laser) at day 15 and the corresponding photos of representative tumor tissues. G) The optical images of representative mice from different treated groups (1) Control; 2) Nano-PT; 3) Laser; 4) Nano-PT + Laser) at a series of time points; tumor sites has been indicated by red circles. Reproduced with permission. [177] Copyright 2018, Wiley.  Reproduced with permission. [56] Copyright 2017, American Chemical Society.

Conclusion and Future Outlook
Undoubtedly, early diagnosis significantly contributes to attaining successful therapeutic interventions. [192] Early diagnosis-especially for cancer-is likely to increase the efficacy of nearly every therapy, ranging from surgery, chemotherapy, radiotherapy to immunotherapy. In addition screening specific diseases' biomarkers (e.g., tumor surface markers), proper surveillance of the influential gasotransmitters would effectively aid disease diagnosis at early stages. [193] Of these, H 2 S is vitally important in a series of signaling pathways associated with various physiological (e.g., antiinflammation and antiapoptosis) and pathological effects (e.g., tumor progress, etc.). [194] Additionally, the high toxicity of H 2 S further emphasizes the importance of monitoring H 2 S, especially for potential air exposures. Currently, several organic probes have been implemented for detecting/imaging H 2 S. However, widespread applicability is restricted by their poor physiochemical conditions, including relatively weak sensitivity and limited circulation. [13,14] Advanced nanomaterials have demonstrated desirable properties as multifunctional platforms for imaging and therapy. [49,195] In recent years, nanomaterials have been continually developed as novel probes for H 2 S-triggered detection, imaging, and therapy ( Figure 1). This review summarizes and discusses all SHTS-based nanomedicines to date, focusing on H 2 S imaging of cancer cells and in tumor-bearing mice as well as for disease therapy (e.g., cancer or I/R injury) ( Table 5). More specifically, various H 2 S imaging approaches using fluorescence, LSPR, UCL, NIR, PA, and PET modalities are summarized. Therapeutic strategies, such as photodynamic and photothermal therapy, influenced by the presences of H 2 S are also discussed in detail. To provide more ideas for the H 2 S related treatments, the H 2 S generated nanoplatforms have been included as well. Undeniably, the development of SHTSbased nanomedicine has seen much progress accelerated by the efforts of researchers. However, there are still several principles and challenges that need to be addressed in future H 2 S-nanoprobe designs. Below we provide a series of considerations regarding these crucial issues for future SHTS-base nanomedicine innovation and translation (Figure 20).

Challenge
Due to physiochemical properties, H 2 S quickly dissolves in water and results in the formation of HS − and S 2− that introduce interference. Additionally, toxic H 2 S generated from cells is processed rapidly by anabolism and catabolism. Due to this dynamic nature, real-time imaging of H 2 S is highly demanded to inform the location/status of disease (e.g., cancer) following therapy. In summary, a specific, sensitive, and multifunctional H 2 S sensor with excellent circulation (for reaching the specific area) is ideal for H 2 S detection and therapy.

Influence of Size, Shape, and Charge
The morphology of nanomaterials, especially size, directly affects the optical features (e.g., LSPR) and contacting area. Both of these aspects are strongly related to the sensitivity toward H 2 S. Additionally, large nanoparticle (>200 nm) tend to absorb more serum proteins (34% absorbance) compared with smaller ones (80 nm, with 6% absorbance). This results in only smaller nanoparticles having a circulation half-life suitable for imaging. [196] Additionally, the nanomaterials biodistribution is significantly affected by their shape and surface charge. [197][198][199] For instance, tumor tissue accumulation is enhanced with negatively charged NPs. [198,199] Thus, varying the diameter, shape, and charge alter biodistribution and tumor penetration and subsequently influence the efficiency of imaging and therapy. [200,201]

Surface Modification
Although the H 2 S detection (e.g., solution, serum or H 2 S in the air) can be performed with unmodified nanomaterials, surface modifications (e.g., PEGlaytion, acetylation, amino acid or ligand/antibody functionalization) greatly increase their stability, biocompatibility, circulation and targeting for in vivo sensing/ delivery. [202,203] Other surface modifications of functional groups or material (e.g., Cyclam-Cu 2+ or FRET acceptor) [45,122] can impart an alternative strategy that affords a specific nanomaterial (such as Au nanorod with photothermal strategy) with H 2 S-selectivity.

Accuracy of Real-Time H 2 S Concentration
During in vivo imaging, interfering background signal from tissue autofluorescence (e.g., skin) greatly affects H 2 S visualization. Although most in vivo NIR or PET imaging agents limit the autofluorescence background, the accuracy of H 2 S detection or imaging would be further influenced by the variation among individuals. As an ideal imaging system, UCNPs can greatly reduce autofluorescence. Additionally, the unique ratiometric strategy applied (i.e., the ratio of specific emission/a control emission) ensure sensing accuracy. Thus, we believe the incorporation of a reference emission using surface modification or reagent loading will increase imaging accuracy during diagnosis and therapy.

Sensitivity Enhancement for In Vivo Imaging
As mentioned above, the biological half-life of H 2 S is short. Typically, biological concentrations are generally lower than the LOD of most nanoagents. To improve the detection performance, an enhancement (e.g, SAM) agent is strongly

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Adv. Sci. 2019, 6,1901724 recommended, especially for H 2 S-triggered therapeutic nanoplatforms. [54] 6.6. Therapeutic Strategy A series of combined therapies including photodynamic, photothermal, and gas-generated treatments, have been listed in this review. These smart nanoplatforms are all H 2 S-regulated and mitigate damage to surrounding tissue. However, the potential problems, including the releasing speed and the concentration of H 2 S generated within a certain area, must be controlled. Meanwhile, additional agents, such as chemical drugs or vaccine adjuvants (e.g., CpG ODN) could be further loaded for combined chemotherapy or immunotherapy after H 2 S activation.

Applications and Selection of Nanosensor
Given diverse applications for SHTS-based nanomedicine, proper nanoplatform selection is critical. For the detection of H 2 S in solution, biosample, and air, the priority of nanosensor selection is the selective, sensitivity, and practicality. For instance, the sensors with a physical supporting (e.g., supporting membrane) or an eye-visible colorimetric examination would be more practical and convenient. Alternatively, biocompatibility and circulation half-life are the key factors for in vivo imaging and therapy. Although great progress has been made in the development of nanomaterials as H 2 S sensors with high sensitivity and selectivity, only a few can apply in the in vivo assay due to the bad biocompatibility and circulation. Thus, to promote the real application of SHTS based nanomedicine and its following clinic translation, more efforts should be dedicated to investigating these aspects.
In a sharp comparison of general strategies, the advances of nanotechnology enable us to combine various functions into one nanoagent. With SHTS-based nanomedicine, we are able to detect and imaging H 2 S for different applications, and also induce specific therapy following the diagnosis. The increasing interest in real-time H 2 S imaging and high performance of SHTS would encourage the further investigation of the following translation in the clinic, which will greatly improve the diagnosis of various H 2 S diagnosis and benefit the patients via a safe and efficient therapeutic strategy.