A Simple Schiff Base Probe for Quintuplicate-Metal Analytes with Four Emission-Wavelength Responses

Abstract

A versatile mono-Schiff compound consisting of o-aminobenzene-hydroxyjulolidine (ABJ-MS) has been easily synthesized using a one-step reaction. ABJ-MS displays four diverse fluorescence responses to the addition of Zn²⁺/Al³⁺/Fe³⁺/Ag⁺, with the maximum fluorescence emission at 530 nm undergoing a hypsochromic shift to 502/490/440/430 nm, synchronously with the discriminating fluorescence enhancement being 10.6/22.8/2.6/7.1-fold, respectively. However, the addition of Cu²⁺ into ABJ-MS leads to an opposite behavior, namely, fluorescence quenching. Meanwhile, ABJ-MS also displays distinct absorption changes after adding these five metal ions due to different binding affinities between them and ABJ-MS, which gives ABJ-MS quite a versatile detecting nature for Cu²⁺/Zn²⁺/Al³⁺/Fe³⁺/Ag⁺. Moreover, ABJ-MS can mimic a series of versatile AND/OR/INH-consisting logic circuits on the basis of the Cu²⁺/Zn²⁺/Al³⁺/Fe³⁺/Ag⁺-mediated diverse optical responses. These will endow the smart ABJ-MS molecule and potential applications in the multi-analysis chemosensory and molecular logic material fields.

1. Introduction

As is well known, metal ions play a quite important role in diverse physiology and industry processes, although they could also cause some serious diseases in excessive/deficient conditions [1,2]. For example, Zn²⁺ as an essential element has taken part in some physiological activities. Yet, the insufficiency/surfeit of zinc ions could cause Friedreich’s ataxia, Alzheimer’s and Parkinson’s diseases [3,4]. Similarly, excessive/deficient Cu²⁺ could bring about certain neurological sickness and gastrointestinal disturbances, Fe³⁺ may induce liver and kidney diseases as well as metabolism maladjustment, while Al³⁺ can cause some damage to human′s central nervous and immune systems [5,6,7,8]. Ag⁺ accumulation could also cause serious consequences, such as grayish-blue skin discolorations [9]. Furthermore, excessive metal ions in domestic/industrial wastewater could induce bad environmental pollution. Thus, developing highly effective methods to detect these metal ions is extraordinarily necessary.

Among numerous analytic techniques, fluorescent probes, particularly those with detecting functions for some important metal ions, are being paid growing attention to due to their rapid responsiveness, on-site real monitoring, and high sensitivity [10,11,12]. However, most fluorescent probes concentrate on one-to-one detections, while only a few studies have focused on one-to-multi-sensing, either simultaneously or separately [13,14,15,16,17]. Nevertheless, the single molecular probes for multi-analytes could not only shorten the preparation procedure for multiple probes but also exhibit a high detecting efficiency compared to single-analyte detecting sensors [18,19,20,21]. Thus, it is extremely desirable to explore and develop single fluorescent probes for multiple targets through diverse optical responses. To actualize the versatile monomolecular detection for multiple targets, generally, more than one luminophore or receptor is combined into the single-molecular probe through a laborious multistep organic synthetic procedure [22,23]. Suzuki and co-workers prepared a probe consisting of tris-fluorophores to detect Fe³⁺/Pb²⁺/Al³⁺/Cu²⁺ based on the diverse complexing modes between receptors and metal ions [24]. Akkaya et al. synthesized a Bodipy probe for Zn²⁺/Ca²⁺/Hg²⁺, consisting of three receptors [25], while the Jiang group prepared a one-receptor-bridging porphyrin-Bodipy dyad probe for Fe²⁺/Hg²⁺ [26]. Meanwhile, few investigations have been conducted on probes consisting of single-fluorophore-receptors for multi-analytes associated with diverse detecting mechanisms, etc. [27,28,29,30]. However, to the best of our knowledge, there still appears to be no exploration into a molecular probe with only one luminophore and a single receptor, except for quintuplicate-metal ions, especially with the easy-to-synthesize process.

Meanwhile, ortho-hydroxy keto–imines are receiving increasing attention owing to their wide application [31,32]. Among them, the salicylaldazine moiety has been one of the most promising construction units for fluorescent probes due to its excellent metal complexing affinity, its unique enol-keto tautomerization, and C=N isomerization, which could induce sensitive optical changes [33,34,35]. In addition, the metal-binding affinity and optical-signal sensibility can be easily improved by the introduction of a third binding site onto the salicylaldazine moiety [36,37]. Herein, we designed and synthesized a new three-in-receptor mono-Shiff molecule (ABJ-MS) by introducing an o-aminobenzene unit onto the salicylaldazine moiety based on the mono-condensation reaction between o-diaminobenzene and 9-formyl-8-hydroxyjulolidine, Scheme 1. As expected, ABJ-MS displays five discriminating fluorescence behaviors with four emission wavelengths after adding Zn²⁺, Cu²⁺, Al³⁺, Fe³⁺, or Ag⁺, which are simultaneously accompanied by the significantly different absorption responses, owing to different binding affinities between these five metal ions and ABJ-MS. In addition, a series of complicated logic circuits consisting of AND/OR/INH gates could be mimicked for the ABJ-MS on the basis of the Cu²⁺/Zn²⁺/Al³⁺/Fe³⁺/Ag⁺-mediated diverse optical responses. These will endow the ABJ-MS molecule and the potential applications in the versatile multi-analysis chemoprobe and the molecular logic material fields.

2. Results

The aimed mono-Schiff probe consisting of o-aminobenzene-hydroxyjulolidine and a three-in-receptor (ABJ-MS) (Scheme 1) was obtained by the mono-condensation of the o-diaminobenzene with the 8-hydroxyjulolidine-9-carboxaldehyde (1:1 equiv.), in ethanol and under a refluxing condition. The ABJ-MS compound was characterized by NMR, elementary analysis, and mass spectroscopies, as shown in Figures S1 and S2 (Supporting Information).

2.1. The Selectivity Ability

To understand the optical properties of ABJ-MS, the electronic absorption and fluorescence emission behaviors of this compound (20 μM) in a DMSO–H₂O mixture with different H₂O fractions (f_w, the volume percentage of H₂O in DMSO-H₂O mixtures) were studied. As shown in Figure S3 (Supporting Information), when the water fraction f_w was below 60%, the electronic absorption of ABJ-MS undergoes minimal changes. Whereas, when f_w was further increased from 60 to 95%, the maximum absorption of ABJ-MS becomes slightly broader and red-shifted, and in the meantime, the level-off tail gradually appears in the long-wavelength region, accompanied by a slight decrease in the absorbance, which is possibly due to the Mie light scattering, and suggests the formation of aggregated nanoparticles. Meanwhile, along with the increase in f_w from 0–60% to 60–5%, the weak fluorescence emission of ABJ-MS was observed to be firstly enhanced and then decreased, probably due to the formation of aggregated nanoparticles, which inhibit the intramolecular rotation motion and enhance the intermolecular π–π stacking interaction, thereby resulting in aggregation-caused quenching in the high f_w conditions [38]. Thus, the sensing properties of ABJ-MS were investigated in a mixture of DMSO/H₂O (v:v = 1:4).

To identify the detecting capability of this compound for metal ions, the optical properties of ABJ-MS (20 μM) were investigated upon the addition of metal ions, such as Zn²⁺, Cu²⁺, Al³⁺, Fe³⁺, Ag⁺, Hg²⁺, Pb²⁺, Co²⁺, Mn²⁺, Ni²⁺, Cd²⁺, Ca²⁺, Ba²⁺, Mg²⁺, Li⁺, Na⁺, or K⁺ (10 equiv.), in DMSO/H₂O (4:1). As can be found from Figure 1, the absorption and fluorescent spectra of ABJ-MS remained almost unchanged after adding the above metal ions, except for Zn²⁺, Al³⁺, Fe³⁺, Ag⁺, or Cu²⁺. After adding Zn²⁺/Al³⁺/Fe³⁺/Ag⁺, the weak fluorescence emission of ABJ-MS displayed diverse responses: The maximum emission wavelength of ABJ-MS (530 nm) underwent a hypsochromica shift to 502/490/440/430 nm, which was synchronously accompanied by a discriminating 10.6/22.8/2.6/7.1-fold increase in fluorescence emission, respectively, alongside an increase in the relative fluorescence quantum yield from 0.005 of free ABJ-MS to 0.040, 0.107, 0.010, and 0.024 upon the addition of Zn²⁺, Al³⁺, Fe³⁺, and Ag+, respectively [39]. On the contrary, the addition of Cu²⁺ induces the fluorescence quenching, Figure 1A. Meanwhile, the addition of these five metal ions into ABJ-MS also causes obviously different changes in absorption spectra. After adding Zn²⁺ ions, the absorption at 427 nm from ABJ-MS undergoes a bathochromic shift to 437 nm following a slight enhancement in absorbance. This is also true for the addition of Cu²⁺, although the maximum absorption appears at 441 nm. Differently, after adding Al³⁺, the absorption of ABJ-MS at 427 nm decreased simultaneously with a bathochromic shift to 436 nm as well as a slightly weak shoulder band appearing around 376 nm. The addition of Ag⁺ also induced a similar change but with the maximum absorption appearing at 373 nm, thereby resulting in the varying ratio of A₄₂₇/A₃₇₃ from 2.8 to 0.7. After adding Fe³⁺, the above-mentioned maximum absorption for ABJ-MS decreased simultaneously alongside an obvious increase at 377 nm, which led to a broad absorption in the range of 310–490 nm, Figure 1B. These results endow ABJ-MS as a quite versatile detecting nature for Zn²⁺/Cu²⁺/Al³⁺/Fe³⁺/Ag⁺ that is associated with five discriminating fluorescence and absorption dual-responses, which is quite rare in reported fluorescence probes.

2.2. The Responsive Metal Sensing

Zn²⁺/Cu²⁺ sensing: The fluorescent titration experiment for ABJ-MS (20 µM) was carried out by increasing the Zn²⁺ content in the DMSO/H₂O (4:1) solution. As can be seen in Figure 2A, along with the increase of Zn²⁺ quantity from 0 to 1.5 equiv., the fluorescence emission from ABJ-MS gets gradually increased with a hypsochromic shift from 530 nm to 502 nm, whereas the strong absorption by this compound is found to undergo a bathochromic shift to 437 nm. Subsequently, the fluorescent emission and absorption spectra remain almost unchanged upon the continuous increase in Zn²⁺ to 10 equiv., Figure S4 (Supporting Information). Furthermore, the fluorescence emission (F − F_min) of ABJ-MS raised linearly along with the increase in Zn²⁺ from 0 to 1.5 equiv., on the basis of the Benesi–Hildebrand equation, Figure 2B, suggesting a probable 1:1 complexing between ABJ-MS and Zn²⁺, where the approximate association constant (K_a) was 3.04 × 10⁴ [40], which is further affirmed in the fluorescent Job’s plot, Figure 2B. In addition, the ESI–mass spectrometry for ABJ-MS upon the addition of Zn²⁺ (10 equiv.) produces a peak at m/z = 388.42, which is assignable to [ABJ-MS + Zn²⁺ + H₂O + H]⁺ (calculated at 388.1), Figure S5 (Supporting Information); thus, again confirming the 1:1 complexing between ABJ-MS and Zn²⁺. Notably, despite the fluorescence quenching response following the addition of Cu²⁺, the fluorescent titration experiment of ABJ-MS (20 µM) with Cu²⁺ (0–10 equiv.) also displays a similar 1:1 binding stoichiometry for the ABJ-MS-Cu²⁺ system, yet with an approximate K_a of 1.2 × 10⁵, Figure 2C,D. The detection limits of ABJ-MS for Zn²⁺ and Cu²⁺ ions were determined as 3.21 × 10⁻⁷ and 1.06 × 10⁻⁷ M under the present condition, which was lower than those applied to the potable water by the WHO [41], suggesting that ABJ-MS could be a highly sensitive probe for Zn²⁺ and Cu²⁺ using the diverse fluorescence off–on and on–off signals, respectively.

Fe³⁺/Al³⁺ sensing: As shown in Figure 3A, the fluorescent titration test for ABJ-MS with Al³⁺ was also studied by successively increasing Al³⁺ (0–10 equiv.) in the DMSO/H₂O mixture (4:1). Alongside increasing the Al³⁺ amount from 0 to 4 equiv., the weak fluorescence emission from ABJ-MS gets gradually increased with a synchronous hypsochromic shift from 530 to 490 nm, resulting in a fluorescence intensity ratio of 25:1 for F₄₉₀/F₅₃₀. Meanwhile, the changes in the absorption for ABJ-MS mainly focus on the added Al³⁺ amount being 0–4 equiv., Figure S6 (Supporting Information). Furthermore, the maximum intensity in the fluorescent Job’s plot for ABJ-MS with Al³⁺ was observed when the molar fraction of (ABJ-MS) vs. (Al³⁺) + (ABJ-MS) was 0.66, suggesting the possible 2:1 complexing of ABJ-MS with Al³⁺, Figure 3B [42]. Based on the 2:1 binding stoichiometry and fluorescence titration experiments of ABJ-MS with Al³⁺, the binding constant for this compound with Al³⁺ was estimated to be 9.52 × 10³, by plotting the Benesi–Hildebrand equation lg[(F_l − F₀)/(F − F₀)] against lg(1/(Al³⁺)) [43,44]. Similarly, the fluorescence titration experiment and fluorescent Job’s plot for ABJ-MS with Fe³⁺ also showed a similar 2:1 binding mode between ABJ-MS and Fe³⁺, with an approximate K_a of 8.72 × 10⁴, Figure 3C,D. The detection limits of ABJ-MS for Al³⁺ and Fe³⁺ ions were determined as 9.07 × 10⁻⁷ and 2.69 × 10⁻⁷ M, respectively, indicating the sensitively diverse fluorescence off–on sensing function of ABJ-MS to Al³⁺/Fe³⁺.

Ag⁺ sensing: As shown in Figure 4A, the fluorescence emission by ABJ-MS at 430 nm gradually increases as the Ag⁺ content increases from 0 to 10 equiv., and the corresponding 1/(F − F_min) raises linearly against the alteration in 1/(Ag⁺) (0–12 equiv.), according to the Benesi–Hildebrand equation, Figure 4B. Moreover, the absorption titration for ABJ-MS with Ag⁺ shows that both absorptions 1/(A − A_min) and 1/(A_max − A) at 373 and 427 nm, respectively, by ABJ-MS also increased linearly against the change of 1/(Ag⁺) (0–10 equiv.), with an approximate K_a of 1.6 × 10³ M⁻¹, Figure 4C,D [45]. These results indicate a possible 1:1 stoichiometry between ABJ-MS and Ag⁺, which is again approved by the fluorescent Job’s plot of ABJ-MS towards Ag⁺, Figure 4A. The detecting limitation by ABJ-MS for Ag⁺ is 1.03 × 10⁻⁵ M, suggesting an excellent detecting function for ABJ-MS towards Ag⁺ through the fluorescence off–on and absorption dual-channels.

2.3. Competition Experiment

The competition experiments were carried out in DMSO/H₂O (4:1) mixtures to further study the detecting abilities for ABJ-MS of Zn²⁺/Cu²⁺/Al³⁺/Fe³⁺/Ag⁺. As can be observed in Figure 5A and Figure S7 (Supporting Information), the fluorescent emission of the ABJ-MS-Fe³⁺ (1:10) complex at 440 nm is quenched after adding Cu²⁺ (10 equiv.); meanwhile, the absorption spectrum is observed to be similar to the ABJ-MS-Cu²⁺ complex, suggesting the substitution of Fe³⁺ by Cu²⁺ in the ABJ-MS–Fe³⁺ composite, which is also true for the ABJ-MS–Zn²⁺/Al³⁺/Ag⁺ complexes. Similarly, the addition of Fe³⁺ into the ABJ-MS–Zn²⁺/Al³⁺/Ag⁺ systems also results in the displacement of metal in these systems by Fe³⁺ as well as the consequential fluorescence response, Figure 5B. In a similar manner, Zn²⁺ can replace the metal ion in the ABJ-MS–Al³⁺/Ag⁺ complex and Al³⁺ can replace the metal ion in the ABJ-MS–Ag⁺ complex, thereby inducing the corresponding fluorescence and absorption responses, Figure 5C,D, Figures S8 and S9 (Supporting Information). By contrast, after adding other metal ions, the fluorescence emission of the ABJ-MS–Cu²⁺/Fe³⁺/Zn²⁺/Al³⁺/Ag⁺ systems remained almost unchanged, Figures S10–S14 (Supporting Information). These results show the excellent binding affinity between ABJ-MS and these five metal ions, in the order of Cu²⁺ > Fe³⁺ > Zn²⁺ > Al³⁺ > Ag⁺, which is in good agreement with their association constant results mentioned, thereby revealing the favorable selectivity of ABJ-MS for Cu²⁺/Fe³⁺/Zn²⁺/Al³⁺/Ag⁺ over other tested metal ions. In addition, the selectivity of ABJ-MS toward anions was also investigated through the addition of sodium salts and different anions, including F⁻, Cl⁻, Br⁻, I⁻, HSO₃⁻, OAc⁻, S⁻², SO₃⁻², SO₄⁻², CO₃⁻², HPO₄⁻², and H₂PO₄⁻. Notably, the electronic absorption and fluorescence emission spectra of ABJ-MS made either a slight or no change upon the addition of 100 equiv. of each anion, meaning little effect was shown by these anions on the sensing properties of ABJ-MS, Figure S15. Thus, ABJ-MS could be utilized as a versatile molecular probe for Cu²⁺/Fe³⁺/Zn²⁺/Al³⁺/Ag⁺ through dual-mode optical signals associated with immediate interactions between these five metal ions and ABJ-MS as well as the diverse metal-displacement of the ABJ-MS–M(Ag⁺/Al³⁺/Zn²⁺/Fe³⁺) complexes by Cu²⁺/Fe³⁺/Zn²⁺/Al³⁺, representing the quite rare example of smart single molecular probe for quintuple-metal analysis using the sensitive fluorescence and absorption dual-signals.

2.4. The Sensing Mechanism

As is well known, there is usually an equilibrium between the enol–imine and keto–enamine isomers for salicylaldazine moiety in solution due to the enol–keto isomerization, while the equilibrium moves easily under trace amounts of acid catalysis [46,47]. To understand the sensing mechanism between ABJ-MS and the corresponding metal ions, the optical behaviors of ABJ-MS after sequentially adding HCl were investigated under the same measuring requirement as mentioned above. As can be found from Figure S16 (Supporting Information), the increase in HCl content from 0 to 1 equiv., caused a bathochromic shift in the strong absorption of ABJ-MS from 427 to 438 nm with a little decrease in the absorbance. Then, continuously increasing the HCl amount, even to 100 equiv., caused the maximum absorption to decrease with a distinct enhancement in the absorption band at 368 nm. At the same time, the fluorescent emission for ABJ-MS at 530 nm was simultaneously decreased alongside the appearance of a new strong emission band at 434 nm along with the increasing HCl amount. The optical responses for ABJ-MS to the addition of HCl can be rationalized as follows: ABJ-MS may coexist as a mixture of enol–imine and keto–enamine isomers, although favors the keto–enamine isomer under a neutral environment, which is approved by the proton band at 13.52 ppm and is associated with an intramolecular hydrogen band (O…H-N) in the ¹H NMR for ABJ-MS [4,30]. Upon the addition of HCl, the H⁺ firstly coordinates with the oxygen atom in the C=O unit from the keto–enamine isomer, which inhibits the intramolecular H-bonding interaction, promotes the conversion from the keto–enamine isomer to the enol–imine isomer, and results in a bathochromic shift in the maximum absorption from ABJ-MS [48,49]. Then, the nitrogen atom from the imine unit is protonated along with the increasing HCl amount, and the photoinduced electron transition is restrained, which in turn induces fluorescence enhancement by ABJ-MS.

Notably, the addition of Al³⁺/Fe³⁺ into ABJ-MS has similar optical responses to those for ABJ-MS under the same acid conditions mentioned above. The responsive optical changes in the ABJ-MS–Al³⁺/Fe³⁺ system may be attributed to the hydrolysis of Al³⁺/Fe³⁺, which results in an acidic environment, promotes the equilibrium shift to the enol–imine form, and increases the complexing of ABJ-MS with Al³⁺/Fe³⁺. These changes, in combination with the 1:2 stoichiometry for the ABJ-MS–Al³⁺/Fe³⁺ complex, imply a possible binding mode between ABJ-MS and Al³⁺/Fe³⁺ ions, Figure 6A [50,51]. For the ABJ-MS–Ag⁺ complex, the quite similar optical responses to the ABJ-MS–HCl system, together with the 1:1 complexing, suggest a possible coordination mode between ABJ-MS and the Ag⁺ ions, Figure 6B. By contrast, upon the addition of Zn²⁺/Cu²⁺, there exist diverse changes in the absorption spectra of ABJ-MS, despite the similar red-shift in the maximum absorption, which could be attributed to the different binding mode for the ABJ-MS–Zn²⁺/Cu²⁺ complex, Figure 6C. The increase in fluorescence by ABJ-MS upon the addition of Zn²⁺/Al³⁺/Fe³⁺/Ag⁺ could be attributed to the restraint of both the C=N isomerization and the excited-state proton transfer (ESPT), in addition to the photoinduced electron transition process [52,53]. However, possibly due to the different characteristic outermost electronic structures between these responsive metal ions, the binding of ABJ-MS to Zn²⁺/Al³⁺/Fe³⁺/Ag⁺ induces a diverse fluorescence emission, whereas its coordination with Cu²⁺ results in fluorescent quenching, which is probably associated with its usual paramagnetic fluorescence quenching property, meaning that the ABJ-MS is quite a versatile multi-responsive molecular probe for Cu²⁺/Fe³⁺/Zn²⁺/Al³⁺/Ag⁺.

2.5. Application of ABJ-MS in Zn²⁺/Al³⁺/Fe³⁺ Analysis in Water Samples

Two kinds of water samples, such as from a river (Yongding River) and artificial water, were employed to explore the practical application potential of ABJ-MS. After performing a simple filtrate, there were no Zn²⁺/Al³⁺/Fe³⁺ according to the atomic absorption spectrometry. The water samples were spiked with a Zn²⁺/Al³⁺/Fe³⁺ solution and analyzed using the ABJ-MS molecular probe. As can be seen from

Table 1. Detection by ABJ-MS of Zn²⁺/Al³⁺/Fe³⁺ in river water samples.

Sample	Zn2+ added (μM)	Zn2+ found(μM)	Recovery (%)	R.S.D. a (%)
River water	0.00	0.00
	10.00	10.06	101	0.6
Artificial water b	0.00	10.05	101	0.5
	10.00	20.28	102	1.4
Sample	Al3+ added (μM)	Al3+ found (μM)	Recovery (%)	R.S.D. a (%)
River water	0.00	0.00
	10.00	10.08	101	0.8
Artificial water c	0.00	10.05	101	0.5
	10.00	20.30	102	1.5
Sample	Fe3+ added (μM)	Fe3+ found (μM)	Recovery (%)	R.S.D. a (%)
River water	0.00	0.00
	10.00	10.08	101	0.8
Artificial water d	0.00	10.05	101	0.5
	10.00	20.30	102	1.5

^a n = 3; R.S.D.: relative stand deviation; ^b–d synthesized by the addition of Zn²⁺/Al³⁺/Fe³⁺ as well as Hg²⁺, Zn²⁺, Ni²⁺, Pb²⁺, Cd²⁺, Ba²⁺, Mg²⁺, Ca²⁺, Li⁺, Na⁺, and K⁺ (10.00 μM) into river water; conditions: 20.00 μM ABJ-MS in mixed solution of DMSO/river water (4:1).

, the results indicated the satisfying recovery and R.S.D. values for the tested samples using this newly prepared probe [54], endowing ABJ-MS with an improved potential application in sensing Zn²⁺, Al³⁺, and Fe³⁺.

2.6. The Construction of the Logic Gate

Inspired by the Cu²⁺/Fe³⁺/Zn²⁺/Al³⁺/Ag⁺ induction of diverse absorption and fluorescence changes, the smart ABJ-MS molecule was employed to construct logic systems. In whole logical operations, the ABJ-MS serviced as a gate, while Zn²⁺/Al³⁺/Fe³⁺/Ag⁺ were utilized as inputs, with the presence and absence being defined as 1 and 0, and the relevant maximum fluorescence emission intensity defined as outputs. Interestingly, a series of different function logic systems could be fabricated for ABJ-MS (20 µM) by rationally defining the logic threshold on the basis of the multiple stimulate-fluorescence signals. For example, when 10 equiv. of Zn²⁺ and Al³⁺ were utilized as inputs (In1 and In2), while the maximum fluorescence emission intensity (Fmax) for ABJ-MS upon the addition of metal ions was defined as outputs, then, two diverse logic gates will be fabricated with different threshold [55,56]: With Fmax at 3000 being the threshold to define the “1” (˃3000) and “0” (˂3000) states, the value of Fmax of ABJ-MS (output) becomes high (F_max = 3580, 1), yet only when the 10 equivalent of Al³⁺ (In2) was added. Conversely, it is at a low level (F_max ˂ 3000, 0) after the addition of Zn²⁺ (In1) as well as a combination of Al³⁺ and Zn²⁺; thus, mimicking an INHIBIT logic gate. Similarly, an OR logic gate was simulated using F_max at 1500 as the threshold, Figure S17 (Supporting Information). Notably, when the four metal ions Ag⁺, Fe³⁺, Al³⁺, and Zn²⁺ were utilized as inputs (In1, In2, In3, and In2) in combination with the two thresholds mentioned above, two complicated logic circuits consisting of AND–OR–NOT and OR–AND–NOT gates were mimicked according to the truth table, Figure 7 and Tables S1 and S2 (Supporting Information). Furthermore, using different Cu²⁺/Zn²⁺/Al³⁺/Fe³⁺/Ag⁺ combinations as inputs and diverse optical responses as outputs, a series of more complicated logic functions for ABJ-MS would be simulated by a rationally designed different threshold. Thus, the smart ABJ-MS fluorescent molecule not only exhibits the quite versatile detecting nature for Cu²⁺/Zn²⁺/Al³⁺/Fe³⁺/Ag⁺ but also could fabricate a series of complicated logic circuits, which endows the small ABJ-MS molecule with a wide potential of applications in the multi-function chemosensory and molecular logic material fields.

3. Materials and Methods

3.1. Chemicals and Instruments

All reactants were commercially available and used without further purification. ¹H and ¹³C NMR spectra (¹H-400 MHz and ¹³C-100 MHz) were recorded on a Bruker DPX 400 MHz spectrometer in CDCl₃ with shifts referenced to SiMe₄ (0.00 ppm). MALDI-TOF mass spectra were recorded by a Bruker BIFLEX III ultra-high-resolution Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer using an alpha-cyano-4-hydroxycinnamic acid as the matrix. Elemental analyses were performed using an Elementar Vavio El III. Electronic absorption spectra were recorded by a U-4100 spectrophotometer. Steady-state fluorescence spectroscopic studies were performed by a F4600 (Hitachi) with a slit width of 5 nm and a photon multiplier voltage of 700 V for emission. The relative fluorescence quantum yields for ABJ-MS and its metal complexes were obtained by comparing the area under the corrected emission spectrum of the test sample with the solution of [N,N-Bis(salicylidene)-1,2-phenylenediamine]zinc(II) in DMSO with an excitation wavelength of 400 nm, which has a quantum efficiency of 0.008, according to the literature [39].

The stock DMSO/H₂O (4:1) solution (1 mM) with either Cu²⁺, Fe³⁺, Hg²⁺, Co²⁺, Mn²⁺, Ni²⁺, Cd²⁺, Ca²⁺, Ba²⁺, Mg²⁺, Li⁺, Na⁺, K⁺, or Zn²⁺ was prepared from their chloride salts, while Pb²⁺ and Ag⁺ were prepared from their nitrate salt, and Al³⁺ from the sulfate salt. For the anion recognition test, stock solutions were prepared in DMSO–H₂O with different anions, NaF, NaCl, NaBr, NaI, NaHSO₃, Na₂S, NaOAc, Na₂SO₃, Na₂SO₄, Na₂CO₃, Na₂HPO₄, or NaH₂PO₄. Meanwhile, ABJ-MS (2 mM) was prepared in DMSO/H₂O (4:1). Testing solution was prepared by placing 0.05 mL of the ABJ-MS stock solutions into volumetric flasks (5 mL), adding a certain amount of tested metal stock, and a constant volume of DMSO/H₂O (4:1), to obtain the required concentration. After mixing for thirty minutes at room temperature, the optical measurements were in progress on the spectrophotometers with a 1 cm standard quartz cell.

3.2. Preparation of Mono-Schiff Probe Consisting of o-Aminobenzene-Hydroxyjulolidine (ABJ-MS)

The compound of o-diaminobenzene (54 mg, 0.5 mmol) was dissolved in anhydrous ethanol (15 mL) before adding 9-formyl-8-hydroxyjulolidine (109 mg, 0.5 mmol), and glacial acetic acid (200 μL). The reacting mixture was heated to reflux under N₂ protection for 5 h before being cooled to room temperature. The orange-yellow suspended substance was filtrated and washed three times in ethanol before the pure ABJ-MS samples were obtained as a yellow solid powder (95 mg, 62%). ¹H NMR (CDCl₃, 400 MHz) δ 13.51 (s, 1H), δ 8.33 (s, 1H), δ 7.25 (s, 1H), δ 7.02 (m, 2H, J = 24 Hz), δ 6.78 (t, 1, J = 16 Hz,), δ 3.96 (s, 2H), δ 3.24 (d, 4H, J = 3.6 Hz), δ 2.78 (m, 4H, J = 31.6 Hz), δ 1.98 (t, 4H, J = 17.6 Hz); ¹³C NMR (100 MHz, CDCl₃) δ161.0, 158.2, 140.7, 140.5, 136.6, 129.8, 126.4, 118.8, 118.1, 115.4, 112.9, 108.9, 106.4, 50.1, 49.8, 27.2, 22.0, 21.1, 20.3; mass spectra (MALDI-TOF): an isotopic cluster peaking at m/z 307.36, (calcd. for (MH)⁺: 307.17); anal. calcd for C₁₉H₂₁N₃O: C: 74.24; H: 6.89; N: 13.67. Found: C: 74.42; H: 6.59; N: 13.71.

4. Conclusions

In summary, a three-in-receptor mono-Schiff probe (ABJ-MS) was developed using a one-step reaction. Upon the addition of Zn²⁺/Cu²⁺/Al³⁺/Fe³⁺/Ag⁺ into the ABJ-MS solution, five distinct fluorescent behaviors with four emission wavelengths were observed simultaneity with differentiable absorption responses due to different binding affinities between these metal ions and ABJ-MS. Furthermore, a series of versatile AND/OR/IN-H-consisting logic circuits could be mimicked for ABJ-MS on the basis of diverse Cu²⁺/Zn²⁺/Al³⁺/Fe³⁺/Ag⁺-mediated optical changes. These show that the smart ABJ-MS molecules are not only quite versatile in detecting Cu²⁺/Zn²⁺/Al³⁺/Fe³⁺/Ag⁺ in nature but also offer great potential in molecular logic fields.