Multi-controlled ternary emission of platinum(II) switches as a visual optical sensor for enzyme and pH detection

Abstract

Near-infrared (NIR) optical sensors are widely applied in biomedical treatment, environmental monitoring, and food safety because of their advantages of avoiding the interference of self-absorption and self-emission. Herein, a pH-sensitive luminescent platinum(II) complex with dual-armed alkyl chains modified with an ammonium ionic head has been designed and explored as an NIR probe. The pH-responsive probe can switch between ternary emission, including green and dual NIR emission, from the monomer, excimer and aggregated ground states in aqueous medium, which can be further regulated by nucleoside polyphosphates via non-covalent interactions with remarkable spectroscopic changes. The time-dependent detection of the assembly/disassembly of an adenosine triphosphate (ATP) and phosphorescent platinum(II) complex can be achieved based on the continuous photophysical changes. Guanosine triphosphate (GTP) can be recognized by visualized dual apparent and emission colors among various nucleoside polyphosphates.

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Introduction

Luminescent sensors have been extensively used to detect special species in the fields of environmental, biological, pharmacological, and physiological science. The advantages of utilizing luminescent probes, including high selectivity and sensitivity, help us to understand the physiological processes and mechanisms in the organism system and to detect harmful species and pollutants in our daily lives. For example, nucleotides are essential biological phosphate molecules that serve as the building blocks of DNA and RNA. In particular, ATP, serving as a primary source of energy in diverse cellular functions, is significant in energy storage in organisms.^1–4 In the ATP molecular structure, the purine base adenine and sugar ribose compose the nucleoside adenosine. Adenosine, in turn, is connected with three phosphate groups through phosphodiester bonds. Each constituent element of ATP, phosphate, adenine, and ribose, plays a different role in intermolecular interactions.^5–7 For biomolecular detection, luminescent probes should not only show high sensitivity but also avoid interferences from other luminescent species in the biological system, because most substances in living systems own high-energy emissive behaviors and yield blue and green emissions. Hence, it is necessary to explore luminescent probes with much longer emission wavelengths.

Some types of luminescent probes based on the emission mechanisms of Förster resonance energy transfer (FRET),^8,9 photoinduced electron transfer (PET),^10,11 intramolecular charge transfer (ICT)^12,13 and excited-state intramolecular proton transfer (ESIPT),¹⁴ have been widely explored to detect environmental and biological species. The optical signal detection method exhibits the advantages of small sample consumption, convenient and rapid operation, and high sensitivity and accuracy. Meanwhile, new and efficient chiral optical sensors based on the electronic absorption spectrum, fluorescence spectrum, circular dichroism spectrum, and circular polarized luminescent spectrum have also attracted much attention. This will provide an effective approach for the separation of chiral materials,^15,16 screening of high-throughput chiral catalytic systems,^17,18 monitoring of environmental contaminants,^19,20 and detection of living entities.^21–24

Phosphorescent complexes with the characteristics of large Stokes shift, long lifetime, and much lower emissive energy are good candidates for optical probes.^25,26 In particular, long-wavelength emission will improve the sensitivity and accuracy due to it reducing or avoiding the interference from luminophores in the microenvironment. Among the various phosphorescent materials, organoplatinum(II) complexes with a square-planar geometry are prone to self-assembly with optical property changes.^27–30 Based on the apparent and emission color changes, it is possible to develop visual optical probes for faster and more intuitive detection.^31–37 We have developed an amphiphilic organoplatinum(II) complex as a volatile organic compound (VOC) sensor.³⁸ This sensor showed a rapid response, high selectivity and good reversibility in water and alcohol vapor with distinct apparent and emissive color changes in the visible light and NIR regions, respectively. The terpyridine platinum(II) complexes can also be explored as a time-dependent probe for monitoring enzymatic activities. Yam and coworkers reported a terpyridine platinum(II) complex with a hydroxyl functional group that exhibited self-assembly properties in an aqueous solution,³⁹ which can be governed by the polyanionic nature of ATP and phosphopeptide. The aggregates were disassembled into adenosine diphosphate (ADP) and inorganic phosphate (Pi) under the catalysis of ATPase with drastic apparent color and emission changes. The discernible alterations in the assembly and hydrolysis processes provide an alternative method to assess the enzyme's activity.

In general, supramolecular assembly systems are often affected by the microenvironment, including factors like temperature, pH and solvents.^40–47 Based on our previous work about alcohol-selective NIR luminescent sensors and molecular hinges with distinct mechano- and piezochromic responses in the NIR region⁴⁸ as well as the visual distinction of the enantiomers due to drastic spectroscopic regulation,⁴⁹ herein, we report a water-soluble cationic cyclometalated platinum complex with double-armed ammonium ions (1) exhibiting emission color changes from green to dual NIR due to switching between the monomer, excimer, and aggregated ground states (Scheme 1). It is designed as an optical probe to accurately detect pH values in a small range. In particular, GTP can be distinguished by the ternary luminescent platinum(II) switch among various phosphate nucleotides, such as adenosine monophosphate (AMP), ADP, ATP, uridine triphosphate (UTP) and GTP. This provides an effective visual approach to detect phosphate nucleotides and avoids interferences from the intrinsic luminescence in organisms.

Scheme 1 Cartoon representation of the acid–base and ATP/ALP (alkaline phosphatase) responsive luminescence and assembly processes.

Results and discussion

Design, synthesis, characterization

The structure of the synthesized complex 1 is shown in Scheme 1. The introduction of a 1,3-di(2-pyridyl)benzene (N^C^N) cyclometalating ligand to the metal complex with the formation of a C–Pt bond can elevate the energy of the nonemissive d–d excited state due to the stronger σ-donating effect of the ligand, resulting in excellent optical characteristics.^50–55 The electron-withdrawing CF₃ substituent on the N^C^N ligand can effectively improve the stability of the complex.^48,56 The introduced double-armed alkyl chain with an ammonium ionic head can increase its solubility in H₂O. More importantly, the positively charged ammonium arms can combine with phosphate anions via electrostatic interactions to induce the assembly of the coordinated platinum(II) complex. The synthetic route of 1, as well as detailed experimental procedures and characterizations, can be found in the ESI (Scheme S1†).

Photophysical and dual-emissive properties

The photophysical properties of complex 1 were first studied in DMF at a concentration of 0.2 mM (Fig. S1A†). The high-energy intensity absorption bands before 350 nm are attributed to the intraligand (IL) [π → π*] transition of the N^C^N ligand. The low-energy, moderately intense absorption bands at ca. 362, 380 and 407 nm originate from the mixed Pt(II)-perturbed IL_Pt [π → π*] transition of the cyclometalated N^C^N ligand and metal-to-ligand charge transfer (MLCT) ([dπ(Pt) → π*(N^C^N)]) transition. The well-resolved vibronic-structured emission bands at 489, 522, and 563 nm, with the vibrational progressional spacings of 1251 and 1395 cm⁻¹ arising from the aromatic C=C and C=N vibrational modes (Fig. S1B†), and a luminescence lifetime at 489 nm of 35 ns (Fig. S2†), primarily originate from the platinum-perturbed ³IL excited state of the N^C^N ligand. A much higher quantum yield in the solid state (21.2%) was obtained than that in solution (0.9%). Notably, the intensity of the NIR emission band at about 700 nm gradually increases without apparent color changes as the concentration increases to 2.0 mM (Fig. S1B†). The emission band at 700 nm is tentatively attributed to the excimer formation of 1 with the evidence of the absorption tail at 425 nm keeping in line with Beer's law (Fig. S3†), which is further supported by the similar excitation spectra of the emission at 489 and 700 nm (Fig. S4†). However, after adding H₂O to the DMF solution, keeping the concentration of 0.4 mM, it was found that the absorption bands at 350–430 nm gradually decreased in intensity and broadened with a slight blue shift as well as the emission color changing from green to orange-yellow as the content of more polar H₂O increased (Fig. 1).

Fig. 1 (A) UV-Vis absorption and (B) emission spectra of 1 (0.4 mM) versus H₂O fraction in the H₂O-DMF mixture (insets: photographs of the emission changes of 1 in DMF and 90% H₂O-DMF solutions); (C) CIE coordinate diagram of the emission color in H₂O-DMF mixtures with different H₂O proportions.

Acid/base responsive ternary luminescence

The double-armed alkyl chain with an ammonium ionic head improves the solubility of the luminescent complex 1 in H₂O with pH = 6.67 measured by a pH meter. A moderately intense absorption band at 380 nm and an absorption tail at wavelength > 500 nm were observed (Fig. 2). Upon addition of KOH or CH₃COOH to the above solution, there were no obvious changes in the UV-Vis absorption and emission spectra with the pH in the range from 6.00 to 8.00 (Fig. S5†). However, a new absorption band at 575 nm appeared accompanied by a drastic apparent color change from orange-yellow to purple as the pH reached 10.60, indicating the formation of aggregates in the ground state with intermolecular π–π stacking and metal–metal interactions. The 575 nm absorption band disappeared and the color changed back to orange-yellow after the addition of CH₃COOH to the above sample with pH = 11.00 (Fig. 2A). Apart from the ³IL green emission, an intense NIR band at 750 nm with an emission shoulder at 825 nm was observed in H₂O. Interestingly, the NIR emission band at 825 nm was increased with the disappearance of the monomer green emission band at 489 nm after the addition of KOH (Fig. 2B). A similar emission spectrum to that of 1 in the initial H₂O solution was obtained after the addition of CH₃COOH. In order to confirm the emission origins, the excitation spectra were further collected. The good overlap of the excitation spectra of the emission peaks at 489, 650 and 700 nm suggests that the 750 nm NIR emission band originates from the excimer formed in the excited state (Fig. S6†). The excitation spectrum of the 820 nm NIR emission band is different from those of the 489, 650 and 700 nm emission bands, in line with the new absorption band at 575 nm, indicating the formation of aggregates in the ground state, resulting in triplet metal–metal-to-ligand charge transfer (³MMLCT). Hence, a reversibly acid/base-responsive ternary luminescence system was constructed based on single small platinum(II) complexes.

Fig. 2 (A) UV-Vis absorption and (B) emission spectra of 1 on addition of base and acid in H₂O solutions (0.2 mM) (insets: photographs of the apparent and emission color on addition of base and acid). (C) UV-Vis absorption and (D) emission spectral changes upon gradual addition of CH₃COOH into the solution of 1 (0.2 mM) (insets: photographs of the apparent color changes of 1 before and after adding CH₃COOH). (E) UV-Vis absorption and (F) emission spectral changes upon gradual addition of KOH into the solution of 1 (0.2 mM).

The above phenomena demonstrate that 1 shows dual visualized apparent and emissive color changes in response to acid and base. The double ammonium ionic heads improve the solubility of the molecules and further suppress the molecular aggregation in H₂O with the appearance of an absorption tail and a weak ³MMLCT emission and the observation of spherical nanoparticles around 100 nm in diameter according to the transmission electron microscopy (TEM) and dynamic light scattering (DLS) results (Fig. 3D and E). Upon addition of a base, the neutrally charged amine groups not only decrease the solubility, but also eliminate the repulsive charge forces and further strengthen the intermolecular π–π stacking and metal–metal interactions resulting in the formation of spherical nanoparticles with enhanced ³MMLCT emission. Subsequently, after the addition of acid, positively charged ammonium ionic heads are reformed and the spectroscopies are also recovered. The possible mechanism is summarized in Scheme 1.

Fig. 3 (A) UV-Vis absorption, (B) photographs of the apparent color, (C) emission spectra and (D) DLS curve variations upon the gradual addition of ATP (0–1.0 eq.) into the Tris-HCl (50 mM) buffer solution of 1 (0.2 mM) (inset A: the absorbance intensity changes at λ = 600 nm; inset C: photographs of the apparent color and emission changes of 1 in 0 eq. and 1.0 eq. solutions). TEM images of (E) 1 (0.2 mM), and (F) 1-ATP (0.2 mM, 0.5 eq. ATP).

Subsequently, we investigated the relationship between pH value and emission. Upon gradual addition of CH₃COOH to the aqueous solution of 1 with a pH value in the range of 2.96–2.34 that inhibits the hydrolysis of the protonated 1, isosbestic and isoemissive points at 434 and 621 nm were observed in the UV-Vis absorption and emission spectra, respectively, suggesting the conversion of the initial hydrolyzed molecules (Fig. 2C and D). The enhancement of the ³IL emission originating from the monomer with decreasing intensity of the NIR excimer and ³MMLCT emission bands is due to the improved solubility and repulsive charge forces, which restrain the aggregates and excimer formation in the ground and excited state, respectively. On the contrary, upon gradual addition of KOH to the aqueous solution of 1 with a pH value in the range of 10.00–11.00, isosbestic and isoemissive points at 506 and 708 nm were observed in the UV-Vis absorption and emission spectra, respectively, along with a drastic apparent color change from yellow to purple, suggesting the complete conversion of the protonated molecules (Fig. 2E and F). The intensity of the NIR ³MMLCT emission band significantly enhanced accompanying the disappearance of the monomer and excimer emission due to the formation of aggregates under strong π–π stacking and metal–metal interactions, caused by the decreased solubility and repulsive charge forces (Fig. 2F and S7†). This indicates that it is possible to precisely regulate the transformation of each individual form of 1 using acid and base (Fig. S8 and S9†). The fluctuations in pH value can be visually and sensitively detected in a small range using molecule 1 with obvious color changes.

ATP-induced supramolecular assembly and dynamic hydrolysis by ALP

Complex 1 modified with an ammonium ionic head and acetate counter ion can provide a microenvironment with pH from 6.00 to 8.00 in H₂O with no obvious changes in the UV-Vis absorption and emission spectra. The complex can provide a near neutral system in which ATP is stable. On the gradual addition of ATP to 1 in aqueous solution, the initial absorption band at 380 nm progressively became much broader, accompanied by the appearance of a new absorption band in the range of 450–750 nm (Fig. 3A). The apparent yellow color turned to purple, which can be conveniently visualized by the naked eye. Similarly, the NIR ³MMLCT emission band at 820 nm was greatly boosted as the emission bands from the monomer and excimer gradually disappeared (Fig. 3C). The above optical behaviors indicate that ATP can induce the assembly of 1 primarily due to the electrostatic interaction between the phosphate and ammonium with a binding ratio of 1 : 2 (ATP : 1), accompanied by the formation of strong π–π stacking and metal–metal interactions yielding the NIR ³MMLCT emission. This is further confirmed by the DLS and TEM results (Fig. 3D–F), which are in line with the disappearance of the signals of the ¹H NMR spectra (Fig. S10†).

ATP is involved in many metabolism-related reactions with the conversion of the triphosphate to di- and monophosphates providing energy to drive many processes in living cells. We used alkaline phosphatase (ALP) as the hydrolytic enzyme to investigate the transformation of 1-ATP aggregates during the ATP hydrolysis process. The UV-Vis absorption spectrum and apparent color showed no obvious changes when ALP was added into the 1 aqueous solution at 37 °C (Fig. S11†). When ALP was added to the 1-ATP aggregates, the intensity of the absorption band at 589 nm was gradually decreased along with an apparent color change from the initial purple color to orange (Fig. 4A). These results indicate that the introduction of ALP hydrolyzes ATP to AMP and Pi, and decomposes the 1-ATP aggregates. This is further supported by the electrospray ionization mass spectrometry (ESI-MS) results with the observation of the AMP species (Fig. S12 and S13†). It was found that the hydrolysis time decreased as the ALP concentration increased according to the absorption intensity monitored at 589 nm (Fig. 4A and B). For example, the hydrolyzed solution with 30 U mL⁻¹ of ALP quickly turned from purple to orange color within 1000 s. The activity of ALP can remain as in the initial state and the hydrolytic reaction can be recycled (Fig. 4B). The disassembly process can also be verified by using CD spectroscopy (Fig. 4C). When ATP is introduced to a 1 aqueous solution, a positive Cotton effect at 500 nm immediately appears, indicating the chiral sense transfer to the nanoparticles. Nevertheless, the CD signal became totally silent in the presence of ALP, further suggesting that an ATP hydrolytic process occurred leading to the dissociation of the 1-ATP assemblies.

Fig. 4 (A) Normalized absorption spectra of ALP with different units. (B) Time-dependent absorbance intensity at 589 nm upon the addition of ATP (0.3 eq.) to 1 (0.2 mM) being repeat several times in the presence of 6, 10, 20 and 30 U mL⁻¹ of ALP. (C) CD spectra of 1, 1-ATP and 1-ATP–ALP in H₂O (medium: 50 mM Tris-HCl, 20 mM MgCl₂).

Selectively sensing phosphated nucleotides

Besides ATP, other phosphate nucleotides, including AMP, ADP, GTP and UTP, were selected to study their optical properties after co-assembly with 1 (Fig. S14†). It is notable that, unlike 1-ADP, 1-ATP and 1-UTP, the UV-Vis absorption spectrum of 1-GTP is similar to that of 1-AMP (Fig. 5A). The sizes of the aggregates as well as their emission properties are related to the negative charge of the phosphated nucleotides (Fig. 5B, C and S15–S22†). 1-AMP showed an emission band at 795 nm originating from the excimer and some mixing ³MMLCT excited state. The emission bands of 1-ADP, 1-ATP and 1-UTP mainly originated from the ³MMLCT excited state. Unusually, GTP completely quenched the emission (Fig. 5B). The phosphate nucleotides GTP and AMP can be conveniently recognized by the visually apparent and emission colors as shown in Fig. 5D. The dual optical detection approach makes the luminescent platinum(II) complexes a good candidate to sense special species and to improve the accuracy.

Fig. 5 (A) UV-Vis absorption, (B) emission spectra and (C) DLS data of 1 (0.2 mM) with different types of phosphates in a Tris-HCl (50 mM) buffer solution. (D) Top: photographs of the apparent color changes of 1 (0.2 mM) with different types of phosphates (0.5 eq.). Bottom: photographs of the emission changes of 1 (0.2 mm) with different types of phosphates (0.5 eq.).

Conclusions

In summary, we have designed and synthesized a water-soluble platinum(II) complex with double-armed alkyl chains modified with an ammonium ionic head. Its ternary emission can be switched between green and NIR colors from the monomer, excimer and aggregated ground states through multilevel assembly. It shows high sensitivity to the pH value and can detect the pH accurately to 0.1 units. The positively charged ammonium arms can combine with phosphate anions via electrostatic interactions to induce the assembly of the luminophores, accompanied by drastic optical behaviors. It can be used to monitor the hydrolytic reaction of phosphate nucleotides. Importantly, the phosphate nucleotide GTP can be conveniently recognized by dual visualized apparent and emission colors. It was demonstrated that the platinum(II) complex with ternary luminescent switching is a good candidate to develop visualized sensors.

Conflicts of interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (22201057 and 21871297), the Zhejiang Provincial Natural Science Foundation of China (LR22B010001 and LQ23B010001), and Hangzhou Normal University (2021QDL001 and 2021QDL065). We sincerely thank Professor Vivian Wing-Wah Yam for her helpful discussion and guidance.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4qi00957f

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