Recognition of Cu²⁺ and Al³⁺ by a Quinolinyl 1,2,3-Triazole Chemosensor: A Comparative Study

Abstract

1,2,3-Triazole units with their structural and photophysical properties are well-suited for the development of chemosensors for ion sensing. Synthetic approaches make it extremely easy to modify this core with just a few steps to control ion selectivity and response-signal output. The current study examines how 8-(4-phenyl-1H-1,2,3-triazol-1-yl)quinoline, a quinoline–triazole–phenyl (QTP) construct, responds differentially to Cu²⁺ and Al³⁺ ions. QTP provides distinct fluorescent signals in acetonitrile in the presence of Cu²⁺ versus Al³⁺, a turn-off response with Cu²⁺ and blue-to-green output with Al³⁺. Spectroscopic studies quantify the selectivity of the sensor for these species with respect to other ions and reveal a stoichiometric ratio of 1:1 for sensor:Cu²⁺ and 2:1 for sensor:Al³⁺. NMR titration studies suggest that Cu²⁺ is detected via coordination of the quinolinyl and triazolyl nitrogens, while Al³⁺ is detected through coordination of the quinoline nitrogen. Overall, QTP displays a selectivity for Al³⁺ relative to Cu²⁺ over other cations in this investigation.

1. Introduction

Cations are of fundamental importance to environmental and physiological processes. While an ion may be essential and advantageous in certain situations, the same ion can be considered xenobiotic or even an indicator of harmful conditions under different circumstances [1]. As such, ion imbalance, including the accumulation of excessive amounts, can have devastating effects.

Copper is important to ATP biosynthesis, proper brain function, and antioxidant processes [2]. Aluminum, the third most abundant metal on earth, is incorporated into materials for several important purposes: for example, water purification systems, food containers, machine parts, and construction [3,4,5]. Over-accumulation of either metal, copper, or aluminum, is harmful. For example, both are linked to the progression of neurodegenerative diseases such as Alzheimer’s and Parkinson’s disease [6]; amassing copper in the liver and brain leads to the debilitating effects of Wilson’s Disease, and the buildup of aluminum in the environment threatens aquatic and plant life [7,8]. For example, both are linked to the progression of neurodegenerative diseases such as Alzheimer’s and Parkinson’s disease [6,9]. Amassing copper in the liver and brain leads to the debilitating effects of Wilson’s Disease, and the buildup of aluminum in the environment threatens aquatic and plant life [10,11]. The development of inexpensive, ion-specific chemosensors to easily detect and quantify these species can facilitate a better understanding of their role in diseases and how they affect the environment [12].

The multifunctionality of the 1,2,3-triazole unit is excellent for sensing applications in colorimetric and fluorometric chemosensors [13]. Photophysical and structural properties of this heterocycle make it suitable for use as a part of the receptor, as a signaling unit, or as a linker connecting binding and reporter sites. As a receptor for cations, triazoles are N-donors through the π-bonded nitrogens [14,15,16]. The triazoles, by virtue of the synthetic procedures used to make them, are relatively easy to modify. The syntheses tend to be tolerant of several functional groups and high-yielding [17,18,19,20]. Facile modification of the substituents on triazoles can serve two important purposes: the ability to dictate (1) the cation selectivity and (2) tune the fluorometric or colorimetric signal output. In the current study, functionalizing the triazole unit with a quinoline moiety provides an extensively conjugated structure that can function as a fluorometric chemosensor for the Cu²⁺ and Al³⁺ ions.

Herein, we report on the contrast in the response of the 8-(4-phenyl-1H-1,2,3-triazol-1-yl)quinoline, a quinoline-triazole-phenyl (QTP) construct to Cu²⁺ versus Al³⁺, two of the ions that induce visible fluorometric changes in the molecule. QTP is known [17] but, to the best of our knowledge, this is the first report on its versatility as a chemosensor. The sensor is made by the coupling of 8-aminoquinoline and acetophenone N-tosylhydrazone in a copper (II)-mediated reaction (Scheme 1) [17]. With extensive conjugation throughout the structure and donor nitrogens in the triazolyl and quinolinyl units, QTP has cation-sensing capabilities that provide clear and distinctly different fluorescence-signal outputs for Cu²⁺ versus Al³⁺ under ultraviolet (UV) illumination at 356 nm—a complete “turn-off” response for Cu²⁺ and a “blue-to-green” output for Al³⁺. Nuclear Magnetic Resonance (NMR), UV-Vis absorption, and fluorescence spectroscopy were used to investigate the interaction of the probe with the cations and to decipher the difference in the binding mode between the two. Our results reveal a stoichiometric ratio of 1:1 for Cu²⁺ and 2:1 for Al³⁺ (QTP:metal).

2. Materials and Methods

2.1. General Experimental

All chemicals and reactants were obtained through commercial sources (Alfa Aeser, Ward Hill, MA 01835, USA; OXCHEM, Dallas, TX 75254, USA; Sigma-Aldrich, St. Louis, MO 63103, USA; Acros, Irvine, CA 92618, USA; and Fisher, Waltham, MA 02451, USA) without further purification. HPLC-grade solvents and de-ionized (DI) water were used for spectroscopic experiments and syntheses. Column chromatography was performed with Selecto Scientific Silica Gel (particle size 100–200 microns).

2.2. Synthesis of 8-(4-Phenyl-1H-1,2,3-triazol-yl)quinoline (QTP)

8-(4-Phenyl-1H-1,2,3-triazol-yl)quinoline (QTP) was synthesized according to a previously reported procedure (Scheme 1) [17]. Structural modifications in QTP resulting from the presence of metal ions were characterized via ¹H-NMR (Nuclear Magnetic Resonance (NMR)) titration experiments. To ensure accurate assignment of the overlapping or shifted proton signals, two-dimensional ¹H-¹H COrrelation SpectroscopY (COSY) was subsequently employed (Figure S1A,B).

2.3. Determination of Stoichiometry (Job’s Plot) Between Sensor and Metal Ions

For Job’s plot, the product of the difference in fluorescence intensity (ΔF) and the mole fraction (Χ), ΔF^.Χ, was plotted against Χ. ΔF is determined from F₀ − F_x, where F₀ and F_x are the fluorescence intensities of QTP in the absence and presence of the metal ions (Mⁿ⁺). The maximum of the plot indicated the mole ratio between the interacting species.

2.4. Calculation of the Limit of Detection (LOD)

The detection limit of QTP for the metal ion, Cu²⁺, was calculated using the fluorescence titration according to the IUPAC definition (Equation (1)). The fluorescence of QTP was measured independently ten times and the standard deviation (σ) of the blank measurement was determined. To find the slope (k), the ratio of fluorescence intensities (F_x/F₀) at 415 nm was plotted against [Cu²⁺].

Limit of Detection (LOD) = 3σ/k

(1)

2.5. Instrumentation

NMR spectra were recorded on an Agilent MR4000DD2 spectrometer (Agilent Technologies, Loveland, CO 80537, USA, originally manufactured by Varian Inc., Palo Atlo, CA 94304, USA) with a multinuclear probe with two RF channels and variable temperature capability, ¹H-NMR: 400 MHz and ¹³C-NMR:100 MHz using deuterated acetonitrile (CD₃CN). Signals were recorded in parts per million (ppm). References in the corresponding solvents were set according to residual CH₃CN at 1.94 ppm for ¹H-NMR and 1.32 ppm [CH₃] and 118.26 [CN] for ¹³C-NMR. Signals for the ¹H-NMR multiplicity are described as: singlet (s), doublet (d), doublet of doublet (dd), triplet (t), multiplet (m), coupling constants (J, Hz) and integration. Melting points were measured with the Vernier Melt Station using Vernier LabQuest 2 and are uncorrected.

Room temperature absorption and steady-state fluorescence measurements were performed using a Shimadzu UV-2450 spectrophotometer (Shimadzu Scientific Instruments, Columbia, MD 21046, USA, manufactured in Kyoto 604-8511, JPN) and a PerkinElmer LS55 (PerkinElmer, Inc., Shelton, CT 06484, USA, manufactured in Buckinghamshire, HP9 2FX, UK) with a well plate reader fluorimeter respectively. Concentrations of QTP were kept at 2.94 × 10⁻⁴ mol dm⁻³ or below in acetonitrile to avoid any possible intermolecular effect. The specific concentration for each experiment is identified in the corresponding figures.

3. Results and Discussion

The differential interactions of QTP with Cu²⁺ and Al³⁺ were investigated through UV illumination, UV–Vis absorption measurements, fluorescence analysis, and NMR spectroscopy. The behavior of the sensor with these cations was compared to a select group of metals to which the probe is essentially nonresponsive under UV-light (Ni²⁺, Cu²⁺, Cd²⁺, Zn²⁺, Li⁺, Al³⁺, Mg²⁺, Ag⁺). These investigations provided valuable insight into the interaction of the chemosensor with Cu²⁺ versus Al³⁺, such as the stoichiometry, the structural motifs involved in binding, and the ion selectivity of the molecule. Owing to the hydrophobic nature of QTP, the sensing mechanism was evaluated using non-coordinating perchlorate salts in organic media. While the probe in its current form is limited for direct aqueous anion-rich environments due to solubility constraints, future work will focus on structural modifications (e.g., appending hydrophilic groups) as also seen in our previous work [16].

3.1. Preliminary Observations Under UV-Light

At a molar ratio of 1:10, QTP: metal salt in acetonitrile, the response of the probe to the series of perchlorate salts of the cations was investigated under UV-light (365 nm) and ambient light (Figure 1a and Figure S2). Clear, visible changes, if any, only occurred under UV-light. The fluorescent blue of the probe was altered by Cu²⁺ and Al³⁺, a complete “turn-off” response with Cu²⁺ and a blue-to-green fluorescence signal with Al³⁺.

To investigate the practical sensing application of chemosensor QTP, paper test strips were prepared by spotting filter paper with QTP (free sensor) followed by the targeted cations (Cu²⁺ and Al³⁺). At 365 nm, the quenching of fluorescence with Cu²⁺ and the green output with Al³⁺ are clearly observed (Figure 1b).

3.2. Fluorescence Spectroscopic Response to Cu²⁺ and Al³⁺ Compared to Other Cations

The changes in the optical output of QTP with Cu²⁺ and Al³⁺ prompted an investigation of the photophysical behavior of the molecule with these ions. Fluorescence spectroscopy revealed that the maximum emission band for QTP occurs at ~411 nm with excitation at the maximum absorption of 294 nm (Figure 2 and Figure S3). Treatment with ten equivalents of the Cu²⁺ perchlorate reduced the intensity of this band by four-fold. The addition of Al³⁺ caused a bathochromic shift, from ~411 nm to ~477 nm, at an intensity that was 2.5-times lower than the original QTP maximum. The other salts enhanced the intensity at ~411 nm by different degrees but, visually, the corresponding response output for those salts was not as distinct as the changes induced by Cu²⁺ and Al³⁺ (Figure 1 and Figure 2).

Overall, screenings with the metal salts under UV-light and fluorescence spectroscopic studies revealed particularly distinct and contrasting responses with Cu²⁺ and Al³⁺. Studies were performed with these two cations to better understand the mechanism by which signaling occurs. All subsequent fluorescence experiments monitored bands at ~411/410 nm for Cu²⁺ and Al³⁺ in addition to ~477 nm for Al³⁺.

3.3. Cu²⁺: UV-Vis Absorption, Fluorescence, and NMR Spectroscopic Investigations

In-depth studies with fluorescence, absorbance and NMR experiments were used to understand the behavior of QTP with Cu²⁺.

3.3.1. Cu²⁺: Investigation by UV-Vis Absorption

In acetonitrile, QTP exhibited an unstructured, low-energy absorption band peaking around 294 nm (Figure 3). Addition of the Cu²⁺ salt resulted in a gradual decrease in the 294 nm absorbance, followed by the emergence of a new band at 315 nm. This modulation in the absorption yielded two isosbestic points at 276 nm and 308 nm (Figure 3). The presence of the isosbestic points which occur due to an equilibrium between different species in a medium supports the formation of a QTP-Cu²⁺ complex. The appearance of the 315 nm band for this new species is due to the charge transfer from the triazolyl nitrogen to Cu²⁺ [21]. An intramolecular charge transfer (ICT) from the fluorophore to the Cu²⁺ occurs when QTP forms a stable complex with the metal ion. Other transitions, such as d-d, π-π*, and charge transfer (LMCT and MLCT) are also responsible for the change in the absorbance spectra of organic molecules with metal ions [22].

3.3.2. Cu²⁺: Fluorescence Spectroscopic Investigation

A fluorescence titration study also revealed a 1:1 binding stoichiometry between Cu²⁺ and QTP (Figure 4 and Figure 5). Changes in the fluorescence at 411 nm were monitored while varying the equivalents of the perchlorate relative to the sensor. The chelation-enhanced fluorescence quenching (CHEQ) impact of paramagnetic Cu²⁺ ions produced a notable decrease in the fluorescence intensity of QTP. For the Job’s plot, the emission intensity was monitored at 411 nm. A non-linear curve fit parameter went through a maximum ΔF^.Χ at Χ = 0.51 correlating with a 1:1 stoichiometric ratio for QTP:Cu²⁺ (Figure 5). A Job’s plot based on the change in absorbance at 315 nm (Figure S4) was consistent with the 1:1 stoichiometry. The binding constant according to the Benesi–Hildebrand plot for this interaction is 1.45 $\times$ 10⁵ M⁻¹ (Figure S10) [23].

In order to understand the type of fluorescence quenching, collisional or dynamic, the decrease in the fluorescence intensity was plotted using the Stern–Volmer equation, F₀/F = 1 + K_SV [Cu²⁺], where F₀ and F are, respectively, the fluorescence intensities of QTP in the absence and presence of the Cu²⁺ (quencher) at 411 nm (Figure 6) [24]. A linear plot indicated dynamic quenching of QTP in the presence of Cu²⁺. K_SV, the Stern–Volmer quenching constant, as revealed from the slope of the plot is 9.26 × 10⁴ M⁻¹ which is higher than the order for diffusion-controlled collisional quenching (10¹–10² M⁻¹) [25,26,27]. This strongly indicated that the quenching of QTP is caused by the complexation with Cu²⁺. The limit of detection (LOD) as revealed from the F_x/F₀ showed a linearity from 1.6 to 24 μM of copper (II) addition. At 1.6 μM, it is therefore possible to detect extremely low levels of Cu²⁺ with QTP using fluorescence spectroscopy. Furthermore, this LOD is in the range of previously reported values, between ~77nM and 2.5 µM, for other small-molecule quinoline-based Cu²⁺-sensors which bind 1:1, sensor:cation [28,29]. On par with other reported LODs, QTP has the added benefit of its one-step synthesis and distinct “turn-off” response with Cu²⁺.

3.3.3. ¹H-NMR Investigations with the QTP-Cu²⁺ Complex

Details regarding the interactions in the Cu-QTP species were obtained using an ¹H-NMR titration experiment with Cu²⁺ where the amount of QTP was held constant. These results clearly showed the participation of the triazolyl and quinolinyl nitrogens in the detection process [30]. Increasing the amount of Cu²⁺ resulted in downfield shifts for the triazole’s H-7 and the quinoline’s H-1 and H-3 (Figure 7). The maximum observable change occurred with H-3, from 8.45 ppm at 0.0 eq of Cu (II) to 8.60 ppm with 0.50 eq of Cu (II). The chemical shifts for H-1 and H-7 seemed to merge at ~9.10 ppm with 0.15 eq Cu²⁺, each originating at 8.99 and 9.04 ppm, respectively. The observed deshielding of protons whose environments depend on the electronic character of the ring nitrogens indicates that both the triazole (H-7) and quinoline (H-1 and H-3) nitrogens participate directly in coordinating to Cu²⁺. The triazolyl–quinolinyl binding-pocket model is strongly reinforced by the pronounced line broadening and eventual disappearance of proton signals. Protons are located in close proximity to the Cu²⁺ center, specifically H-2 and H-6 at 0.15 equiv. of Cu(II), followed by H-1 and H-7 at approximately 0.30 eq consistent with direct paramagnetic perturbation upon coordination [31]. Not much could be discerned at ≥0.75 eq due to broadening of the signals caused by the paramagnetism of Cu²⁺. However, the information obtained at lower equivalents clearly demonstrates the involvement of N-donor groups from both the quinoline and triazole in the formation of a probe-metal complex (Scheme 2).

3.4. Al³⁺: UV-Vis Absorption, Fluorescence, and NMR Spectroscopic Investigations

Spectroscopic investigations with fluorescence, UV-Vis absorption and NMR revealed the mechanism of response to Al³⁺.

3.4.1. Al³⁺: Investigation by UV-Vis Absorption

The spectrophotometric titrations of QTP were performed with different concentrations of Al³⁺ salt in acetonitrile (Figure S5). Upon addition of Al³⁺, the absorption bands centered around 294 nm showed a gradual decrease in absorbance with the simultaneous development of a new peak around 345 nm. In addition to these changes, the high-energy band (~294 nm) showed red-shifted absorption. The preliminary observation of QTP with Al³⁺ in UV-Vis absorption prompted a quantitative analysis using fluorescence spectroscopy for the formation of the QTP-Al³⁺ complex.

3.4.2. Al³⁺: Fluorescence Spectroscopic Investigation

As mentioned in the fluorescence screening study (Section 3.2), the emission of QTP yielded a peak around 411 nm. Fluorescence spectroscopy titration experiments with the QTP sensor and Al³⁺ showed a decrease in the emission band at ~411 nm until 0.3 eq of the metal with respect to QTP (Figure 8). Development of a new band around 477 nm was observed when 0.4 equivalents of Al³⁺ were added to QTP. This is consistent with the signal output observed under UV-light in Figure 1 which shows that Al³⁺ induces a blue to green color change with QTP. The ratiometric change in the emission of QTP is considered an outcome of the combined effect of the Lewis acidity of Al³⁺, solvation and the formation of the metal-ligand complex [32]. Quinoline probes have often shown modulation in the emission due to changes in local pH [33]. Asthana et al. [34] synthesized a potent fluorescent sensor, CMO for the detection of Al³⁺. Further, the CMO-Al³⁺ ensemble demonstrated effective performance as a selective probe for the detection of pyrophosphate (PPi) and picric acid. The results from their study revealed that the operational pH range of CMO for the detection of Al ³⁺ is 4–8.

Job’s plot based on the emission band intensity at 411 nm revealed a 2:1 stoichiometric ratio for the sensor: Al³⁺ ion (Figure 9). In this plot, the non-linear curve fit parameter went through a maximum ΔF^.Χ at Χ = 0.70, correlating a 2:1 binding stoichiometry between QTP and Al³⁺. The binding constant based on the Benesi–Hildebrand plot is 1.42 $\times$ 10⁶ M⁻² (Figure S11) [23]. The LOD based on the Stern–Volmer plot is 4.27 µM (Figure S6) somewhat higher than similar small-molecule sensors which bind 2:1, sensor:Al³⁺ with LODs in the ~70–80 nM range [35,36]. However, as noted previously, QTP’s one-step synthesis and its distinct response to the cation, in this case blue to green, are an advantage; this speaks to its potential for further development for practical use.

3.4.3. ¹H-NMR Investigations with the QTP-Al³⁺ Complex

NMR titration experiment with QTP and aluminum perchlorate strongly suggests that the sensor binds to Al³⁺ via its quinolinyl nitrogen (Figure 10) [37,38,39]. The behavior of the sensor with this ion is vastly different from that of Cu²⁺ (Figure 7). The change in the triazole proton’s resonance, H-7, is marginal compared to quinolinyl signals that are substantially shifted downfield between 0 and ~0.50 eq of Al³⁺ (Figure 10). With increasing amounts of Al³⁺, the resonances for H-1 and H-3 move from 8.99 ppm to 9.22 ppm, and 8.46 ppm to 9.32 ppm, respectively. The chemical shift for H-5 changes from 7.79 ppm to 8.18 ppm while signals for H-2 move from 7.65 ppm to 8.23 ppm. The peaks for H-4 at 8.24 ppm and H-6 at 8.14 ppm eventually merge and become centered around 8.50 ppm. In contrast, the triazole’s C_sp2-H resonance, H-7, moves upfield by only 0.06 ppm from 9.04 ppm to 8.98 ppm. This slight upfield shift as opposed to a significant downfield movement in the H-7 signal strongly suggests that the triazolyl nitrogens are not involved in the binding to the Al³⁺ [38,39]. The phenyl signals, H-8, H-9 and H-10, remain unchanged regardless of the amount of Al³⁺ and after 0.6 eq, the quinolinyl signals stop responding to the addition of the cation. A Job’s plot based on the NMR titration corroborated the results of the fluorescence Job’s plot, with a maximum of ~0.7 that indicates a ratio of 2:1, QTP:Al³⁺ (Scheme 3, Figure 9 and Figure S7).

DFT calculations on the QTP–Al³⁺ coordinated geometries were performed using Gaussian 09W software with the ωB97XD functional and 6-311G(d,p) basis set using acetonitrile as solvent (Figures S8 and S9) [40]. The solvation was computed using the SMD (Solvation Model based on Density) model [41]. The optimized geometry suggests the participation of triazole nitrogen along with the quinolinyl nitrogen in binding Al³⁺ with QTP molecules as evidenced by their close proximities (Figures S8B and S9). In the optimized geometry, the close proximity of nitrogen atoms to Al³⁺, especially N₂₁—Al₆₇ and N₁₅—Al₆₇ points towards a nitrogen-assisted binding rather than an isolated one (Figure S9). NMR titration with Al³⁺ on QTP though shows little change in the triazole signal but this is not explicit of the excluded triazole nitrogen atom in binding with Al³⁺. Triazole nitrogen coordinated to Al³⁺ can induce localized electronic perturbations which might have little effect on the NMR signals as evidenced by the DFT calculations suggesting the triazole nitrogen and quinolinyl nitrogen bound QTP–Al³⁺ structure [42].

3.5. Interference Studies Probing the QTP’s Response to Cu²⁺ and Al³⁺ via Fluorescence

Interference studies to determine the sensitivity of the sensor compared to the other cations in this investigation were performed with Cu²⁺ and Al³⁺ cations.

3.5.1. Interference Studies with Cu²⁺

To varying degrees, five equivalents of Cu²⁺ muted the fluorescence of the sensor in the presence of ten equivalents of the Ni²⁺, Cd²⁺, Zn²⁺, Mg²⁺, Li⁺, and Ag⁺ ions (Figure 11). Significantly, in all cases, fluorescence intensity was reduced to 60% of the original value or less with the addition of copper (Figure 11). The most pronounced effect was observed with Ni²⁺ and Ag⁺ with which the addition of copper (II) reduced the fluorescence by more than 80%.

3.5.2. Interference Studies with Al³⁺

For the aluminum perchlorate addition of 5 eq. of Al³⁺ to an acetonitrile solution of the sensor and 10 eq. of the interfering cations, Ni²⁺, Cd²⁺, Zn²⁺, Mg²⁺, Li⁺, and Ag⁺, showed a clear reduction in the intensity at ~410 nm (Figure 12). A comparison of the fluorescence output at 410 nm in the presence and absence of Al³⁺ indicates a distinct and strong preference for the Al³⁺ ion over the other metals, more so than was observed with Cu²⁺. In all cases, the 410 nm band was reduced to ~30% or less of its original value when Al³⁺ was added.

3.5.3. Cu²⁺ Versus Al³⁺ Interference

In exploring the preference of the sensor for Al³⁺ compared to Cu²⁺_, it was noted that a selectivity for Al³⁺ was somewhat maintained in the presence of Cu²⁺. The addition of 5 eq of the copper (II) perchlorate salt to a solution of the sensor and 10 eq of Al³⁺ reduced the intensity at 477 nm to ~80% of the initial value but did not revert to ~411 nm maximum that is observed when the sensor is only in the presence of Cu²⁺ (Figure 2 and Figure 13a). Doing the study in reverse, treating a mixture of the sensor and 10 eq. copper (II) with half the amount of Al³⁺, 5 eq., caused a red shift in the λ_max. from ~411 nm to ~460 nm. This movement toward the 477 nm band clearly shows a response that is specific to Al³⁺ and is most indicative of the aluminum cation’s ability to compete with Cu²⁺ for binding to QTP.

4. Conclusions

Under UV illumination (365 nm), QTP, a triazolyl-quinoline-based sensor, was found to produce a “turn off” response to Cu²⁺ and blue-to-green signal output for Al³⁺. NMR and DFT studies provided valuable information about the binding interaction. QTP appears to interact with Cu²⁺ via nitrogens in both the triazole and quinoline units based on NMR data while Al³⁺ strongly interacts with the molecule through the quinolinyl nitrogen as seen in NMR, and through the triazole N as predicted by DFT calculations. Job’s plots from fluorescence titration studies confirm a 1:1 stoichiometric ratio for QTP:Cu²⁺ and a 2:1 binding ratio for QTP:Al³⁺; each result is corroborated by two separate experiments from the following: NMR, absorbance or fluorescence studies. Overall, the sensor displays a marked selectivity for Al³⁺ over other ions explored in this study, including Cu²⁺. Results from this work provide rich insight into the binding mode and selectivity of the sensor. These findings will establish that the triazolyl–quinolinyl scaffold forms a well-defined, metal-selective binding pocket capable of differentiating Cu²⁺ from Al³⁺, positioning this platform as a strong candidate for next-generation metal-ion sensors, chelators, and coordination-driven functional materials. Our group is currently focusing on substituted pyridine and quinoline ion sensors. The scope of this manuscript is restricted to the design and evaluation of QTP with Cu²⁺ and Al³⁺ in comparison to a narrow selection of non-response-inducing cations. Consequently, to further develop this quinoline scaffold for practical applications, the impact on the photophysical response with a wider selection of cations, variation in the counter anions, and different conditions that mimic environmental pH will be investigated in future iterations of this work [43].