Exploring multi-stimuli-responsive Pt(II) complexes: supramolecular self-assembly, lysosome-specific targeted photodynamic therapy and photodegradation of organic pollutants

Abstract

Developing novel luminescent materials with multi-stimuli-responsiveness and multi-functional properties is vital. Herein, we designed and synthesized aggregation-induced emission (AIE)-based platinum(II) compounds, Pt-Cl and Pt-PF, with multi-stimuli responsiveness. Pt-PF exhibited excellent AIE properties and featured high sensitivity to changes in solvent conditions. Moreover, Pt-PF self-assembled in water through the aggregation of Pt–Pt and π–π stacking interactions. Owing to its remarkable AIE properties, Pt-PF served as a unique imaging probe for lysosomes, demonstrating excellent photostability. Notably, Pt-Cl and Pt-PF exhibited high reactive oxygen species generation capabilities and excellent biocompatibility, making them superior photosensitizers for photodynamic therapy treatment against cancer cells by targeting lysosomes. Additionally, the Pt-Cl photocatalyst efficiently photodegraded organic pollutants with a high efficiency of 98.7% under white light irradiation for only 60 min. These results provide valuable guidance for designing multi-functional molecules and developing stimuli-responsive materials.

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Introduction

With the rapid growth of the global economy, the photoelectric industry has witnessed significant development. Notably, organic luminogens have gained increasing significance and are widely used in organic light-emitting diodes,^1–5 fluorescence sensors,^6–9 and biological imaging.^10–19 Therefore, it is vital to design and synthesize novel types of organic luminogens, explore their properties under different external stimuli, and apply them across various fields. However, researchers have mainly focused on enhancing the performance of developed organic luminogens in specific fields, often overlooking their potential applications. Consequently, reports on multi-stimuli-responsive organic luminogens are few. Tang et al. reported a D–A triphenyl-boron-modified triphenylamine compound and investigated its applications in electroluminescence, electrochromism, thermochromism, mechanochromism, and bioimaging.²⁰ This “simple” molecule has vital applications in numerous fields, introducing a new idea for developing multi-stimuli-responsive organic luminogens. Moreover, Yuan et al. designed and prepared a multifunctional fluorescent molecule using fluorenone molecules. This fluorescent molecule can be used for detecting fluorescence, observing mechanochromic responses, and imaging biological molecules.¹⁹ Therefore, developing new multi-stimuli-responsive organic luminogens to harness the properties of these materials and address various applications is vital.

Organometallic complex materials, integrating the capabilities of metals (oxidation–reduction and optical activity) with the mechanical and solution processing properties of organic molecules, have received increasing attention and are widely used in various academic fields, such as energy, electronics, and biomedicine.^21–27 However, in most organic fluorescent materials, coordination with metals tends to result in a decrease in fluorescence or quenching, which significantly limits the application and development of organometallic fluorescent materials.^28–31 Among various luminescent materials, luminophores with aggregation-induced emission (AIE) properties exhibit unique optical properties.^32–42 In dilute solutions, AIE luminogens (AIEgens) are weakly emissive or non-emissive, while in the solid or aggregated state, AIEgens become highly emissive. Hence, the concept of AIE might provide a new direction for designing organometallic complex fluorescent materials. Particularly, square-planar platinum(II) complexes have emerged as a vital class of stimuli-responsive luminescent molecular solids. The ligand-to-ligand charge transfer (LLCT), metal-to-ligand charge transfer (MLCT), and metal-to-metal–ligand charge transfer (MMLCT) can induce strong emissions from the visible to the near-infrared region.⁴³ Additionally, the strong supramolecular Pt–Pt and/or π–π interactions contribute to the multi-stimuli responsiveness of square-planar platinum(II) complexes. These complexes are widely used in supramolecular self-assembly, organic photoelectric devices, biological imaging, and photodynamic therapy (PDT).^44–49 However, the development of AIEgen-based platinum(II) square-planar complexes with multi-stimuli responsiveness in a molecule has been rarely reported.

In this study, we designed and synthesized platinum(II) bzimpy complexes, Pt-Cl and Pt-PF, through anion exchange. The optical properties of Pt-Cl and Pt-PF were extensively investigated, revealing their distinct AIE properties. Moreover, the planar molecular structure of Pt-PF facilitated MMLCT self-assembly through Pt–Pt and π–π interactions. Owing to their good emission and metallic properties, Pt-Cl and Pt-PF were used for lysosome-specific targeted imaging. Additionally, Pt-Cl and Pt-PF exhibited excellent high photostability, a high capability for generating reactive oxygen species (ROS), and excellent biocompatibility, making them superior photosensitizers for both PDT treatment of cancer cells and the photodegradation of organic pollutants. This study provides valuable guidance for developing new multifunctional and multi-stimulus organometallic complex materials.

Experimental

Reagents and materials

The solvents used during the synthesis were purified by standard procedures. Pyridine-2,6-dicarboxylic acid, o-phenylenediamine, N-bromooctane, and dipotassium tetrachloroplatinate (K₂PtCl₄) were purchased from Energy Chemical and used without purification. All the other chemicals were obtained from local commercial suppliers and used without further purification.

Characterization

¹H and ¹³C NMR spectra were obtained on a Bruker AV 400 spectrometer in an appropriately deuterated solution at room temperature. High resolution mass spectra were obtained on a Thermo Scientific LTQ Orbitrap XL. UV–vis absorption spectra were obtained on a Shimadzu UV-2600 spectrophotometer. Photoluminescence (PL) spectra, absolute fluorescence quantum yields and fluorescence life-times were recorded on a steady state and transient state fluorescence spectrometer of Edinburgh FLS 980 or 1000. Powder X-ray diffraction was conducted on a SMART APEX II CCD. Thermogravimetric analysis (TGA), differential scanning calorimetry (DSC) and differential thermogravimetry (DTG) were performed on a NETZSCH STA449F3. Scanning electron microscope (SEM) were measured on a ZEISS Sigma 500.

Synthesis

Synthesis of the ligand 2,6-bis(1H-benzo[d]imidazol-2-yl)pyridine (L1). Under argon protection, pyridine-2,6-dicarboxylic acid (1.67 g, 10 mmol), o-phenylenediamine (2.16 g, 20 mmol) and phosphoric acid (20 mL) were added into a 100 mL round bottomed flask. The mixtures were stirred and refluxed for 6 h at 230 °C. After cooling to room temperature, the reaction solution was poured into 500 mL of 4–6 °C ice water and a blue precipitate appeared immediately. The blue precipitate was obtained through filtering. Then the precipitate was added to 150 mL hot K₂CO₃ solution with stirring and filtered to obtain a pink solid. Finally, using methanol recrystallization a light pink solid product named L1 (1.84 g, 59% yield) was obtained. ¹H NMR (400 MHz, DMSO-d₆) δ 8.36 (d, J = 7.8 Hz, 2H), 8.19 (dd, J = 8.1, 7.5 Hz, 1H), 7.78 (dd, J = 5.6, 3.5 Hz, 4H), 7.32 (dd, J = 5.9, 3.1 Hz, 4H). ¹³C NMR (101 MHz, DMSO-d₆) δ 151.03, 148.24, 139.55, 123.36, 121.77. HRMS (ESI) m/z calcd for C₁₉H₁₄N₅⁺ (M + H)⁺ 312.12437, found 312.12488.

Synthesis of ligand 2,6-bis(1-octyl-1H-benzo[d]imidazol-2-yl)pyridine (L2). Ligand L1 (5.0 g, 16 mmol) and sodium hydroxide (1.54 g, 64 mmol) were added into a three-necked flask and purged with argon three times. Then N-bromooctane (13.95 mL, 80 mmol) and N,N-dimethylformamide (100 mL) were slowly added to the mixture using a syringe. The resulting mixture was stirred and refluxed for 12 h at 100 °C. At the end of the reaction, the solution was extracted three times with dichloromethane (DCM). The organic phase was dried with anhydrous Na₂SO₄ and the solvent was removed under reduced pressure. The crude product was purified by silica gel column chromatography (DCM/methanol = 50 : 1) to afford the desired product as yellow oil named L2 (5.14 g, 60% yield). ¹H NMR (400 MHz, CDCl₃) δ 8.33 (d, J = 7.8 Hz, 2H), 8.06 (t, J = 7.9 Hz, 1H), 7.88 (d, J = 7.2 Hz, 2H), 7.47 (d, J = 7.8 Hz, 2H), 7.40–7.30 (m, 4H), 4.71 (t, J = 7.4 Hz, 4H), 1.73 (dd, J = 13.8, 6.8 Hz, 4H), 1.15–0.93 (m, 20H), 0.76 (t, J = 7.2 Hz, 6H). ¹³C NMR (101 MHz, CDCl₃) δ 150.18, 150.02, 142.79, 138.14, 136.27, 125.49, 123.49, 122.74, 120.31, 110.36, 44.91, 31.55, 30.08, 29.04, 29.02, 26.68, 22.50, 14.00. HRMS (ESI) m/z calcd for C₃₅H₄₆N₅⁺ (M + H)⁺ 536.37477, found 536.37518.

Synthesis of complexes Pt-Cl. Ligand L2 (5.14 g, 9.59 mmol), K₂PtCl₄ (4.38 g, 10.54 mmol) and dimethyl sulfoxide (120 mL) were added into a round bottomed flask. Then the flask was purged with argon three times and heated under refluxing for 10 h at 90 °C. After cooling, the mixture was evaporated under reduced pressure to remove excess dimethyl sulfoxide. Then the solution was extracted three times with DCM. The organic phase was dried with anhydrous Na₂SO₄. The crude product was purified by silica gel column chromatography (DCM/methanol = 50 : 1) to afford the desired product as red powder named Pt-Cl (5.50 g, 71% yield). ¹H NMR (400 MHz, DMSO-d₆) δ 8.55 (t, J = 8.1 Hz, 1H), 8.26 (d, J = 8.3 Hz, 2H), 7.77 (d, J = 8.3 Hz, 2H), 7.64 (d, J = 8.5 Hz, 2H), 7.42 (t, J = 7.7 Hz, 2H), 7.25 (t, J = 7.7 Hz, 2H), 4.45 (d, J = 6.7 Hz, 4H), 1.86–1.70 (m, 4H), 1.35 (d, J = 28.1 Hz, 4H), 1.25 (s, 16H), 0.85 (t, J = 6.5 Hz, 6H). ¹³C NMR (101 MHz, DMSO-d₆) δ 152.53, 147.55, 143.49, 138.70, 134.21, 126.97, 126.51, 124.68, 116.10, 113.07, 45.49, 31.61, 30.13, 29.08, 28.99, 26.18, 22.57, 14.43. HRMS (ESI) m/z calcd for C₃₅H₄₅N₅Pt⁺ (M)⁺ 765.30057, found 765.30078.

Synthesis of complexes Pt-PF. To a round bottomed flask (100 mL), Pt-Cl (0.15 g, 0.187 mmol), excess potassium hexafluorophosphate (0.30 g, 1.63 mmol) and methanol (100 mL) were charged under air. On stirring for 30 min at room temperature, a red precipitate was produced. It was filtered to collect the red sediment. It was then dissolved in DCM and the solvent was removed by vacuum distillation to obtain the red product Pt-PF (0.17 g, 98% yield). ¹H NMR (400 MHz, DMSO-d₆) δ 8.57 (t, J = 8.1 Hz, 1H), 8.26 (d, J = 8.3 Hz, 2H), 7.65 (d, J = 8.5 Hz, 4H), 7.43 (t, J = 7.7 Hz, 2H), 7.20 (t, J = 7.7 Hz, 2H), 4.42 (s, 4H), 1.79 (s, 4H), 1.44 (s, 4H), 1.33 (t, J = 7.6 Hz, 16H), 0.96–0.86 (m, 6H) ¹³C NMR (101 MHz, DMSO-d₆) δ 152.65, 147.70, 143.46, 138.76, 134.28, 127.03, 126.58, 124.61, 116.25, 113.01, 45.46, 31.61, 30.11, 29.08, 29.00, 26.20, 22.56, 14.43. HRMS (ESI) m/z calcd for C₃₅H₄₅ClN₅Pt⁺ (M)⁺ 765.30057, found 765.30103.

Cell cultures and colocalization imaging

The U2OS cells were cultured in Dulbecco's modified Eagle's medium (DMEM) supplemented with 10% fetal bovine serum (FBS) and 1% penicillin–streptomycin at 37 °C under a humidified atmosphere of 5% CO₂. The culture medium was replaced every 2–3 days, and when the cells reached 80–90% confluence, they were harvested using 0.25% trypsin-EDTA. To visualize the localization of Pt-PF in U2OS cells and investigate its colocalization with lysosomes, we grew 105 cells in 35 mm glass-bottomed confocal culture dishes and incubated them at 37 °C and 5% CO₂ for 24 hours. The cells were then treated with 5 μM Pt-PF and 1 μM LysoTracker Green in culture medium for 1 hour at 37 °C in 5% CO₂. After removing the medium, the cells were rinsed three times with phosphate-buffered saline (PBS) and imaged under a confocal microscope, with Pt-PF fluorescence detected in the red channel. Pt-PF in the red channel (λ_ex = 405 nm, λ_em = 600–720 nm); LysoTracker Green in the green channel (λ_ex = 488 nm, λ_em = 490–511 nm).

Photo-stability of Pt-PF and LysoTracker Green

To assess the photostability of Pt-PF and LysoTracker Green in U2OS cells, we used a laser with excitation at 405 nm and a low power of 3% to minimize photobleaching. We performed 15 consecutive scans, each lasting 80 seconds, for a total imaging time of 20 minutes. The imaging was carried out using a confocal microscope and the resulting images were processed using ZEN 3.6 software.

Intracellular ROS generation detection

To investigate the effect of Pt-PF on U2OS cell oxidative stress under light irradiation, we divided the cells into two groups. The first group was treated with complete medium containing Pt-PF at a concentration of 5 μM, while the second group was treated with complete medium without Pt-PF. After incubating for 1 hour, both groups of cells were irradiated under white light at an intensity of 38 mW cm⁻² for 1 hour. After irradiation, the cells were washed three times with PBS and then incubated with DCFH-DA at a concentration of 1 μM for 30 minutes. DCFH-DA is a fluorescent dye that is widely used to detect intracellular reactive oxygen species (ROS). The cells were then imaged using a confocal microscope. DCFH-DA in green channel (λ_ex = 488 nm, λ_em = 520–562 nm).

Live/dead cell imaging of Pt-PF treated U2OS Cells

To investigate the combined effect of Pt-PF and the CA/PI combination on U2OS cell viability, we co-incubated the cells with Pt-PF at a concentration of 5 μM and CA/PI at a concentration of 1 μM each for 1 hour. Cells were imaged using a confocal microscope under two conditions: with light exposure and without light exposure, respectively. Calcein-AM in the green channel (λ_ex = 488 nm, λ_em = 491–520 nm). PI in red channel (λ_ex = 488 nm, λ_em = 623–662 nm).

Cell counting kit-8 (CCK-8) cytotoxicity assay

To evaluate the toxicity of Pt-PF towards osteosarcoma cells, we performed the standard CCK-8 assay. Approximately 1 × 10⁴ cells were seeded in each well of a 96-well plate and incubated overnight in 100 μL of culture medium. The medium was then replaced with 100 μL of fresh medium containing different concentrations of Pt-PF (ranging from 0 to 15 μM) and incubated for 24 hours. Some samples were incubated in the dark, while others were exposed to white light at an intensity of 38 mW cm⁻² for 1 hour. After 24 hours of incubation, 10 μL of CCK-8 solution was added to each well containing 90 μL of fresh medium and incubated for an additional 2 hours. The absorbance at 450 nm was measured using a multifunctional micro plate reader (SpectraMax®i13x) to determine cell viability. Each test was conducted in six replicate wells.

Photodegradation properties of Pt-Cl

The photodegradation performance of Pt-Cl was evaluated with a xenon lamp in the wavelength range of 365–780 nm and 300 W as follows: 20 mg of Pt-Cl were added to rhodamine B aqueous solution (40 mg L⁻¹), and the distance between the xenon lamp and the solution was 15 cm. Under magnetic agitation, the mixture solution was exposed to a xenon lamp for 90 minutes. The suspension was removed and centrifuged at 10 000 rpm for solid–liquid separation. The solution was analysed using a UV–vis spectrometer. The same process was used for the determination of photodegradation methylene blue and tetracycline hydrochloride.

Results and discussion

Aggregation-induced emission and supramolecular self-assembly

Pt-Cl was easily synthesized through a three-step process with a high yield of 71%, and the synthetic route is shown in Scheme S1 (ESI†). Subsequently, Pt-PF was obtained through anion exchange (Scheme 1). The products, including ligands L1 and L2, Pt-Cl, and Pt-PF were characterized via proton nuclear magnetic resonance (NMR) spectroscopy and high-resolution mass spectroscopy (HRMS). Detailed information is available in the ESI (Fig. S1–S12).†Pt-Cl and Pt-PF exhibited good solubility and high stability in common organic solvents, including degassed DCM, tetrahydrofuran (THF), and dimethyl sulfoxide (DMSO). The thermal properties of Pt-Cl and Pt-PF were assessed via TGA, DSC and DTG (Fig. S13 and S14, ESI†) under a N₂ atmosphere, revealing high thermal stability. The corresponding data are summarized in Table 1.

Scheme 1 Structure of platinum(II) complexes Pt-Cl and Pt-PF.

Table 1 Spectroscopic data for compounds Pt-Cl and Pt-PF

	Absorption, λabc^a (nm)	Emission						T 3% (°C)
		λ em (nm)	τ soln (us)	τ s (us)	Φ soln (%)	Φ agg (%)	Φ s (%)
a In degassed DCM, with c = 1.0 × 10^−5 mol L^−1. b Fluorescence lifetime in DCM solution. c Fluorescence lifetime in the solid state. d Fluorescence quantum yield in DCM solution. e Fluorescence quantum yield in DMSO/water (v/v = 10/90) solution. f Fluorescence quantum yield in the solid state.
Pt-Cl	318, 358, 431	573/652	0.513	0.533	3.43	3.85	23.6	242
Pt-PF	319, 360, 464	573/652	0.847	10.30	1.28	16.6	28.4	320

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Furthermore, the photophysical properties of Pt-Cl and Pt-PF in the solution and aggregate states were recorded (Table 1). The UV–vis spectra (Fig. 1 and Fig. S15, ESI†) of Pt-Cl and Pt-PF exhibited similar absorption bands within the range of 260–530 nm. The strong, high-energy absorption with large extinction coefficients (log ε > 10⁴ M⁻¹ cm⁻¹) in the range of 260–400 nm was attributed to the spin-allowed ligand-centered (LC) π–π* transition of the bzimpy ligands and alkynyl ligands. The low-energy absorption bands were identified as a combination of MLCT [dp(Pt)-p*(bzimpy)], LLCT [p(alkynyl)-p*(bzimpy)] and LLCT [p(alkynyl)-p*(bzimpy)] transitions. Moreover, the PL spectra of Pt-Cl and Pt-PF exhibited high luminescence in DCM solutions and displayed a well resolved vibronic-structured emission band in their emission spectra with a band in the range of ca. 530–800 nm (Table 1). Thereafter, the luminescence properties of Pt-Cl and Pt-PF in the solid state were also investigated (Fig. 1B). Pt-Cl and Pt-PF exhibited high fluorescence quantum yields (Φ_F = 23.6% and 28.4%, respectively) and red light-emission (680 and 700 nm, respectively) (Table 1). Furthermore, the absorption and PL spectra of Pt-PF in different solvents were investigated to evaluate its ground and excited states (Fig. S16 and Table S1, ESI†). Notably, Pt-PF displayed slightly different absorption peaks, indicating that the optical properties of Pt-PF were insensitive to solvent polarity in the ground state (Fig. S16A, ESI†). However, Pt-PF was non-emissive in solvents, except for DCM, possibly owing to the rapid torsion of the alkyl chain (Fig. S16B, ESI†).

Fig. 1 (A) Normalized absorption spectra and photoluminescence (PL) spectra of Pt-Cl measured in DCM. (B) Normalized PL spectra of Pt-Cl and Pt-PF in the solid state. Pt-Cl concentration: 10 μM.

According to these results, the optical characteristics of Pt-PF in DMSO with different water contents (f_w) were investigated. In a pure DMSO solution, Pt-PF exhibited a pale-yellow color with absorption bands ranging from 260 to 530 nm (Fig. 2A). However, Pt-PF featured strong solvatochromic changes in a DMSO/water mixture. With increasing f_w, the color of the Pt-PF solution significantly changed from pale-yellow (100% DMSO) to red (70% water). The absorption spectra of Pt-PF in different DMSO/water compositions revealed that the LC π–π* transition absorption bands at 260–530 nm, observed in the pale-yellow solution in 100% DMSO, decreased as f_w increased up to 70% (Fig. 2A). Upon the addition of water, a new MMLCT absorption band emerged in the range of 490–620 nm, causing an immediate color change in the Pt-PF solution from pale-yellow to red (Fig. 2B). As f_w increased, stronger Pt–Pt and π–π stacking interactions occurred, resulting in a lower MMLCT energy. Consequently, red shifts were observed in the absorption spectra of Pt-PF, indicating the formation of aggregate species through stronger Pt–Pt and π–π stacking interactions in the presence of water. Furthermore, alongside the significant changes in the absorption spectrum of Pt-PF, the PL spectrum underwent significant changes owing to the alterations in the Pt–Pt and π–π stacking interactions as the water content increased. Notably, Pt-PF exhibited excellent AIE properties and displayed significant emission behaviors in DMSO/H₂O mixtures as f_w increased (Fig. 2C). In pure DMSO solutions, Pt-PF was non-emissive (Φ_F = 0.11%) and displayed a constant emission efficiency at f_w of ≤50 vol%. However, at f_w > 60 vol%, the peak fluorescence intensity of the Pt-PF peak at 660 nm significantly increased. At f_w of 95 vol%, Pt-PF featured a ∼1400-fold increase in PL intensity (Φ_F = 16.6%) compared with Pt-PF in the pure DMSO solution (Fig. 2D). Moreover, the fluorescence images of Pt-PF in different f_w values under a UV lamp further illustrated the AIE behavior of Pt-PF, consistent with the observations in Fig. 2C. Importantly, this obvious absorbance and emission change can also be observed in the Pt-Cl system (Fig. S17, ESI†). The absorption and PL spectra revealed that Pt-PF and Pt-Cl were assembled through the Pt–Pt and π–π interactions.⁵⁰ Meanwhile, the self-assembly phenomenon of Pt-PF and Pt-Cl has been demonstrated by scanning electron microscopy (SEM) and dynamic light scattering (DLS) measurements (Fig. 3 and Fig. S18, ESI†). It can be seen from SEM that the morphology of Pt-PF changed from nanoparticle-like (Fig. 3A) to rod-like (Fig. 3B). When the water content was increased up to 90%, Pt-PF self-assembled to form a mesh filamentous morphology (Fig. 3C). Furthermore, DLS shows that the size of Pt-PF increased with the increasing of the content of water (Fig. 3D). Consequently, the MMLCT process of Pt-PF and Pt-Cl induced self-assembly and excellent AIE behaviors as f_w increased.

Fig. 2 Solutions of Pt-PF in DMSO/water mixture (percentage of water in DMSO from top to bottom: 0, 10, 20, 30, 40, 50, 60, 70, 80, 90, 95%). (A) Absorption spectra of Pt-PF (50 μM); (C) PL spectra of Pt-PF (10 μM) upon increasing the water content in DMSO; Plot of absorbance (B) at 570 nm and PL intensity (D) at 660 nm versus percentage of water.

Fig. 3 SEM images of Pt-PF at different contents of water (A) 0%; (B) 40%; (C) 90%; (D) DLS of Pt-PF at different contents of water.

Lysosome-specific targeted and photodynamic therapy of cancer cells

Photodynamic therapy (PDT) is a potent approach widely used for both diagnosing and treating cancer. Therefore, developing new photosensitizers for PDT is vital. Organometallic compounds play a vital role as photosensitizers and have been widely used and reported. The Pt-PF complexes exhibited high emission and excellent AIE properties. Moreover, the potential application of Pt-PF in biological imaging was investigated.

First, osteosarcoma cells (U2OS) were incubated with 5 μM Pt-PF for 1 h. Subsequently, the cell microscopic imaging was observed via confocal laser scanning microscopy (Zeiss LSM980). Distinct red emissions from lysosomes were observed under a fluorescence microscope (Fig. 4A). To confirm the lysosome-targeting ability of Pt-PF, co-staining experiments were conducted using LysoTracker Green DND-26 (a commercially available probe specifically designed for targeting lysosomes) (Fig. 4B). The granular lysosomes were significantly visible in the red channel, exhibiting complete overlap with the green fluorescence of LysoTracker Green DND-26 and yielding a Mander coefficient of up to 0.8611 (Fig. 4C–F). This indicates the accumulation and staining of Pt-PF in lysosomes, making it suitable for targeted cancer cell imaging. Additionally, the photostability of Pt-PF was evaluated through continuous scanning using a confocal microscope. Fig. S19 (ESI†) indicates the fluorescence intensity of Pt-PF and LysoTracker green in U2OS cells. Compared with LysoTracker green, the emission of Pt-PF was hardly changed during 15 consecutive uninterrupted scans over a 20 min period, confirming the excellent photostability of Pt-PF.

Fig. 4 CLSM images indicate the colocalization of Pt-PF (5 μM) with commercial lysosome-staining agent LysoTracker Green (1 μM) in U2OS cancer cells for 1 h. (A) Pt-PF fluorescence; (B) fluorescence of LysoTracker Green; (C) an overlay image of parts (A) and (B); (D) DIC image; (E) an overlay image of parts (A), (B) and (D); (F) colocalization Mander's coefficient: 0.8611, scale bar: 30 μm.

Lysosomes, critical subcellular organelles present in most eukaryotic cells, were vital targets for PDT, owing to the potential for cell death resulting from lysosomal damage. The selectivity of Pt-PF for lysosomes and AIEgens in generating ROS upon aggregation was explored to further investigate its application in photodynamics. To evaluate the ROS generation ability of Pt-PF, the commercial indicator 2,7-dichlorohydrofluorescein diacetate (DCFH-DA) was used as a probe. Under only 10 mW cm⁻² white light irradiation, the presence of Pt-PF significantly enhanced the PL intensity of DCFH-DA compared with the absence of Pt-PF (Fig. 5A). This result indicates that Pt-PF exhibited higher ROS generation capacity. To further investigate the potential of Pt-PF as a photosensitizer for PDT, cells were co-incubated with Pt-PF and DCFH-DA in the dark for 1 h, followed by white light irradiation at a power density of 38 mW cm⁻² (Fig. 5B–E). Compared to conditions without Pt-PF, the co-incubation of Pt-PF and DCFH-DA led to a significantly increased green fluorescence signal in cells, particularly in the presence of DCFH-DA. This observation indicated the effectiveness of Pt-PF in generating ROS within living cells upon light irradiation.

Fig. 5 (A) ROS generation of Pt-PF and Pt-Cl upon laser irradiation using DCFH-DA (1 μM) as an indicator; detection of intracellular ROS generation in U2OS cells: (B) and (C) incubation with DCFH-DA for 1 h followed by irradiation with white light irradiation for 1 h; (D) and (E) incubation with Pt-PF (5 μM) and DCFH-DA for 1 h followed by irradiation with white light irradiation for 1 h, scale bar: 50 μm.

Furthermore, to assess the effectiveness of Pt-PF-mediated PDT on U2OS cells, we evaluated the impact of Pt-PF on cell viability using calcein-AM and propidium iodide (PI) staining (Fig. S20, ESI†). U2OS cells were co-incubated with Pt-PF and the calcein-AM/PI staining kits for 1 h. The control cells featured a green signal of calcein-AM before exposure to light irradiation, indicating excellent biocompatibility of Pt-PF under dark conditions (Fig. S20A and S20B, ESI†). However, upon white light irradiation (38 mW cm⁻²) for 1 h, the experimental group exhibited a significant red fluorescence, while the green fluorescence signal was barely detectable (Fig. S20C and S20D, ESI†). These results suggest that Pt-PF featured remarkable potential for PDT in cancer cells. Moreover, we evaluated the photo-induced cytotoxicity of Pt-PF on U2OS cells using the CCK-8 assay (Fig. 6). Under dark conditions, Pt-PF exhibited negligible cytotoxicity, indicating its excellent biocompatibility. In contrast, after 1 h of white light irradiation, the cytotoxic effect of Pt-PF was concentration-dependent, indicating its strong photodynamic effect on tumor cells. These results revealed that Pt-PF can serve as alternative photosensitizers for PDT.

Fig. 6 Cell viability of U2OS cells stained with different concentrations of Pt-PF in the absence (dark) or presence (light) of white light irradiation for 1 h.

Photodegradation of organic pollutants

Currently, numerous photocatalytically active materials, including inorganic, organometallic complexes, and metal-free materials, have been extensively studied for pollutant decomposition owing to their good stability, high catalytic activity, and favorable environmental characteristics. Owing to the higher ROS generation capacities of Pt-PF and Pt-Cl, the photocatalytic activity of Pt-Cl was evaluated. Under visible light irradiation, rhodamine B (RhB) was used as a model organic pollutant to assess the photodegradation ability of Pt-Cl, and the photodegradation behaviors were demonstrated using absorption spectra.

The RhB aqueous solutions were stable under visible light irradiation.⁵¹ However, in the presence of Pt-Cl, the color of RhB significantly changed from pink to colorless, indicating the possible degradation of RhB after photocatalysis. Moreover, the characteristic peak (530 nm) in the absorption spectra of RhB was nearly nonexistent within 90 min (Fig. 7A). With increasing Pt-Cl concentration, the degradation rate of RhB was significantly enhanced (Fig. 7B). At a Pt-Cl concentration of 20 mg, the degradation of RhB was completed in only 60 min, achieving a high degradation ratio of 98.7%. Significantly, the photodegradation of Pt-Cl can be repeated for at least four cycles without any variation in efficiency (Fig. S21, ESI†). This indicates a higher photodegradation efficiency of Pt-Cl compared with common inorganic photocatalysts, such as C₃N₄, TiO₂, and some heterogeneous systems.⁵¹

Fig. 7 UV–vis spectra to monitor the process of photocatalytic degradation of RhB (A), MB (C), and tetracycline (D); (B) Relative absorption intensity plot for the photo-degradation of RhB in the presence of different Pt-Cl in aqueous solution. Pt-Cl: 20 mg.

To demonstrate the photodegradation of RhB, the ¹H NMR spectra of RhB before and after light exposure were examined (Fig. 8). After irradiation, the ¹H NMR spectra of RhB exhibited nonexistent characteristic proton chemical peaks. These proton peaks occurred in a high-field region, indicating the complete decomposition of RhB. Additionally, the red residue of Pt-Cl post-irradiation was filtered, collected, and then characterized via¹H NMR (Fig. 8). We observed that the proton chemical shift of Pt-Cl remained nearly similar to that of the original Pt-Cl, indicating the high stability of the photocatalyst. To elucidate the photodegradation effect of Pt-Cl, two other water pollutants, methylene blue (MB) and tetracycline, were also tested. Under irradiation, the characteristic absorption peaks of MB and tetracycline significantly decreased, and the color of the solutions changed from blue to colorless for MB and from colorless to pink for tetracycline (Fig. 7B and C). MB and tetracycline exhibited degradation rates of 73.56% and 56.89%, respectively. This indicates the significant role of Pt-Cl in the photodegradation of these organic pollutants.

Fig. 8 ¹H NMR spectrum (400 MHz, D₂O and DMSO-d₆) of RhB and Pt-Cl before and after white light irradiation.

Conclusions

In conclusion, we designed and synthesized multi-stimuli-responsive AIE-based platinum(II) compounds, Pt-Cl and Pt-PF. Pt-PF and Pt-Cl exhibited a pale-yellow color and weak fluorescence in pure DMSO solvent. However, upon the addition of a small amount of water to the Pt-PF or Pt-Cl solution, Pt-PF and Pt-Cl self-assembled through the aggregation of Pt–Pt and π–π stacking interactions. This resulted in a sensitive color change of Pt-PF and Pt-Cl from yellow to red and a significant enhancement in fluorescence intensity, confirming the AIE activity of Pt-PF and Pt-Cl. Owing to its remarkable AIE properties, Pt-PF served as a fluorescent probe for imaging lysosomes in U2OS cells, demonstrating its excellent photostability. Moreover, the unique metal activity of Pt-PF contributed to its excellent ROS generation efficiency. Regarding both fluorescence emission and ROS generation abilities, Pt-PF exhibited excellent biocompatibility and a strong PDT treatment effect. The use of Pt-Cl as a photocatalyst for investigating an efficient photodegradation method for organic pollutants (RhB) revealed its enhanced photocatalytic ability. Pt-Cl exhibited high efficiency (98.7%) in the photodegradation of organic pollutants under white light irradiation for only 60 min. Therefore, Pt-PF and Pt-Cl are promising and superior candidates for use as photosensitizers for PDT and photocatalysis. This study provides valuable guidance for designing multi-stimuli responsive organometallic complexes with AIE properties.

Author contributions

Hui Ding, Xiepeng Deng and Xiang Liu: experiments and writing–original draft preparation. Wenzhao Shang, Sufan Wang, Xia Liu and Xiangrong Chen: formal analysis, validation and investigation. Xianchao Du, Huifang Su and Hongjian Liu: project administration, funding acquisition, and writing–review and editing. All authors have read and agreed to the published version of the manuscript.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work is financially supported by the National Natural Science Foundation of China (81902356, 82072581, 82272842, and 81972523), the Doctoral Program of Nanyang Normal University (2022ZX006), the National Natural Science Cultivation Fund of Nanyang Normal University (2023PY013). The Henan Province Young and Middle-Aged Health Sciences and Technology Innovation Talent Project (YXKC2022032), the Excellent Young Scientist Foundation of Henan Province (232300421053), and the Program for Science & Technology's Innovation Talents in Universities of Henan Province (24HASTIT066). We thank the Translational Medical Center, First Affiliated Hospital of Zhengzhou University for technical support.

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Footnotes

† Electronic supplementary information (ESI) available: Details of the synthesis; structural information for the compound (NMR and mass spectra). See DOI: https://doi.org/10.1039/d3qi02584e

‡ These authors contributed equally to this work.

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