A near infrared fluorescent dye for trivalent ions sensing and working as a molecular keypad lock

Abstract

A near infrared fluorescent dye (DNSA-SQ) with internal charge transfer (ICT) properties is presented for selectively screening trivalent ions (Al³⁺/Fe³⁺/Cr³⁺) with a ratiometric fluorescence change over other metal ions. A 2 : 1 stoichiometric binding mode between the dye and the trivalent ions was confirmed. Moreover, DNSA-SQ prefers to interact with monovalent Ag⁺ ions in the presence of a protein, resulting in a fluorescence enhancement. Using a synergistic effect, DNSA-SQ also behaves as a molecular keypad lock with sequential chemical inputs of BSA and Ag⁺ ions.

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Introduction

Trivalent ions such as Al³⁺, Fe³⁺ and Cr³⁺ play important roles in living systems and show a pronounced impact on the environment and human health. For example, Al³⁺ is well-known for its abundant existence in nature. It is toxic to humans and causes drinking water contamination. Many diseases, such as Alzheimer's disease and osteoporosis, possibly derive from aluminium toxicity. Al³⁺ also interferes with Ca²⁺ metabolism, decreasing liver and kidney functions.¹ Fe³⁺ is the most abundant transition metal in cellular systems and has wide importance given its presence in a variety of enzymes and proteins. Fe³⁺ is easily involved in electron transfer reactions and oxygen transportation due to its electron-deficient characteristic.² Cr³⁺ is an essential element in human nutrition and plays the pivotal role of activating certain enzymes and stabilizing proteins and nucleic acids. A deficiency in Cr³⁺ can result in a lipid metabolism blockage. At the same time, environmental pollutants lead to concern in industry and agriculture.³ Over the past few years, many fluorescent sensors have been reported for the selective detection of pollutants due to their high sensitivity, simple operation and real-time image.^4–6 Compared to divalent cation sensors, the number of fluorescent sensors, especially with “turn-on” responses, for the detection of Al³⁺, Fe³⁺ and Cr³⁺ is few.⁷ Furthermore, single sensors which can simultaneously detect all of these trivalent ions are scarcely reported.^7,8 Usually, fluorescence detection at a single wavelength is easily influenced by variations in the environment. By contrast, ratiometric fluorescent sensors employing the measurement of emission intensities at two different wavelengths could effectively eliminate the environmental effects and provide a built-in correction.⁹ Therefore, the design of ratiometric fluorescent sensors capable of simultaneously detecting different trivalent cations is currently of great interest.

In addition, silver ions, which are heavy metal ions, are known to inactivate sulfhydryl enzymes and bind with amines, imidazoles and carboxyl groups of various metabolites, resulting in a negative impact on organisms due to their bioaccumulation.¹⁰ Fluorescent sensors for Ag⁺ sensing often suffer from interference from the existence of other heavy metal ions.¹¹ Improving the chelating capability for Ag⁺ by introducing more additional binding units is potentially a way to differentiate Ag⁺ from other relative ions.

As part of our interest into the development of fluorescent sensing systems, we report herein a simple sensor, DNSA-SQ, based on a dansyl amide (DNSA)-substituted squaraine (SQ) derivative for the detection of the trivalent cations Al³⁺, Fe³⁺ and Cr³⁺ (Scheme 1). Upon addition of the trivalent cations, an obvious ratiometric fluorescence change of DNSA-SQ in CH₃CN was observed. Furthermore, this simple sensor can further switch its fluorescence selectivity to Ag⁺ with bovine serum albumin (BSA) also present in the sensing medium. In virtue of the additional binding sites on the cavity of the protein, a “turn-on” fluorescence response to Ag⁺ was observed. Using a synergistic effect, an interesting electronic logic device mimicked a molecular keypad lock has been set up.

Scheme 1 Structure of DNSA-SQ.

Experimental

General

BSA was purchased from Xiaan Wolsen Bio. Reagents Co. (Xiaan, China). All chemicals and reagents were used directly as obtained commercially unless otherwise noted. The salts used in the stock solutions of the metal ions were ZnCl₂, CaCl₂, MgSO₄, KNO₃, Ni(NO₃)₂·6H₂O, AgNO₃, Co(OAc)₂·4H₂O, Mn(OAc)₂·4H₂O, Cu(NO₃)₂·3H₂O, Cd(OAc)₂·6H₂O, FeCl₂·4H₂O, CrCl₃, Hg(OAc)₂, Pb(OAc)₂·3H₂O, Al(ClO₄)₃·9H₂O, FeCl₃·6H₂O and AlCl₃. The water used was ultra filter deionized water. The squaraine dye, DNSA-SQ, was synthesized and purified as reported previously.¹²

Measurements

The absorption and emission spectra were collected using a Shimadzu 1750 UV-visible spectrometer and a RF-5301 fluorescence spectrometer (Japan), respectively. NMR spectra were collected on a Varian 300 Gemini spectrometer.

Sample preparations and titration experiments

Stock solutions of the metal ions and BSA were prepared in deionized water. The concentrations were fixed at 1.0 × 10⁻² M. A stock solution of DNSA-SQ (5.0 × 10⁻⁴ M) was prepared in CH₃CN and further diluted to 5.0 × 10⁻⁶ M for the titration experiments. UV-vis and fluorescence spectra were monitored within 15 seconds.

Results and discussion

Absorption and fluorescence responses to Al³⁺ in CH₃CN

The absorption spectrum of the probe, DNSA-SQ, in CH₃CN exhibited a main absorption peak at 663 nm. Upon addition of Al(ClO₄)₃, the absorption intensity at 663 nm gradually decreased and simultaneously a new absorption peak appeared at 625 nm with a well-defined isosbestic point at 639 nm (Fig. 1a). Accordingly, upon excitation at 620 nm and the successive addition of Al(ClO₄)₃, the maximum emission peak of DNSA-SQ at 681 nm decreased and a new emission peak at 641 nm appeared with the fluorescence intensity increasing. The emission intensity change levelled off when the amount of Al(ClO₄)₃ added was 4 equivalents compared to the probe (Fig. 1b). The ratio of the emission intensities at 641 and 681 nm (I₆₄₁/I₆₈₁) went from 0.088 in the absence of Al(ClO₄)₃ to 5.30 in the presence of 4 equivalents of Al(ClO₄)₃, which is a 60.2 times increase. In the ¹H NMR titration spectra, the complexation appeared to be complete when 0.5 equivalents of Al³⁺ was added. The addition of more than 0.5 equivalents of Al³⁺ caused no further spectral changes (Fig. 2), supporting the fact that the DNSA-SQ–Al³⁺ complex formed with a 2 : 1 stoichiometry. The presence of solvent coordination with the metal ions resulted in higher equivalents (4 equiv.) of Al³⁺ than the exact stoichiometry to saturate the fluorescence intensity.^7a,8c,10 The fluorescence titration measurements showed that the binding constant was 6.06 × 10⁴ M⁻².

Fig. 1 UV-vis (a) and fluorescence (b) spectra of the probe DNSA-SQ (5 μM) upon the addition of Al(ClO₄)₃ (0–10 equiv.) in CH₃CN (λ_ex = 620 nm).

Fig. 2 ¹H NMR titration spectra of DNSA-SQ with Al(ClO₄)₃ in CDCl₃ and methanol-d₄ (v/v = 1/3).

The same changes in the absorption and fluorescence spectra were also observed upon the addition of AlCl₃ (Fig. S1, ESI†). Compared to Al(ClO₄)₃, AlCl₃ caused a weak ratiometric fluorescence change, and more AlCl₃ was needed to reach spectral saturation. It is proposed that the stronger coordinating ability of the Cl⁻ anions compared with the ClO₄⁻ anions towards Al³⁺ weakens the chelating ability of Al³⁺ towards DNSA-SQ.¹⁴ The 2 : 1 stoichiometry (DNSA-SQ : Al³⁺) and the association constant (6.0 × 10⁵ M⁻²) were estimated using the Benesi–Hildebrand method with the fluorescence titration data (Fig. S2, ESI†).¹³ More direct evidence was obtained from the ESI mass spectrum (Fig. S6, ESI†), where the ion at m/z 485.15 corresponded to the molecular ion peak of [2(DNSA-SQ) + Al³⁺ + 2CH₃CN + H₂O]/3 (calculated as 485.12). A linear relationship between the ratio of the emission intensities (I₆₄₁/I₆₈₁) and the Al³⁺ concentration in the range of 20–100 μM was observed, indicating that DNSA-SQ can accurately measure the quantity of Al³⁺ with a detection limit (3σ/slope) as low as 0.9 μM (Fig. 3). More interestingly, the same fluorescence changes were also achieved by the addition of both FeCl₃ and CrCl₃ under identical conditions (Fig. S4 and 5, ESI†).

Fig. 3 The fluorescence ratio (I₆₄₁/I₆₈₁) of the probe DNSA-SQ (5 μM) was linearly related to the concentration of Al³⁺ (20–100 μM).

Selectivity to trivalent metal ions and the proposed recognition mechanism

Surveying other metal ions, monovalent and divalent ions showed no fluorescence response to DNSA-SQ, except that Cu²⁺ induced a 4-fold fluorescence quenching. As a paramagnetic metal ion, Cu²⁺ quenches most fluorogens.¹⁵ The ratiometric fluorescence change means that trivalent metal ions can be readily distinguished from M⁺ and M²⁺ ions without interference from Cu²⁺ (Fig. 4 and 5). An intriguing question was present: why does DNSA-SQ show a selective ratiometric recognition for trivalent metal ions? DNSA-SQ features an internal charge transfer (ICT), where the electron donor and acceptor are located on the dimethylamine phenyl and the squaraine plane, respectively. Coordination of the dimethylamine group with trivalent ions should inhibit the internal charge transfer (ICT), resulting in a blue-shifted emission band.¹⁶ The existence of an isosbestic point for M³⁺ in the fluorescence titration spectra verifies that the fluorescence signal with a short wavelength came from the species DNSA-SQ + M³⁺. For Al³⁺, the fluorescence intensity reached a plateau above 25 equivalents. However for Fe³⁺ and Cr³⁺, the fluorescence intensity continued to increase until 28 and 36 equivalents had been added, respectively. The presence of a fast equilibrium between the free metal ions and the solvated metal ions makes the metal ions less accessible to the dye, thereby requiring more than the theoretical stoichiometric amounts of the metal ions for full intensity saturation.¹ From the ¹H NMR titration spectra, the peak for the methyl protons on the dimethylamine phenyl group at 2.83 ppm is divided into two peaks [2.83 and 2.96 ppm (Δδ = 0.13 Hz)] upon addition of Al³⁺ in DMSO-d₆. This result suggests that the dimethylamine group participates in the coordination with Al³⁺, which is consistent with an ICT-induced blue-shift spectrum change. Al³⁺ coordination also triggers a clear downfield shift of the hydrogen atoms in the benzene group as Al³⁺ chelation reduces the electron density on the backbone of molecules. Thereby, a plausible 2 : 1 binding mode is presented in Scheme 2. Addition of EDTA to the solutions of DNSA-SQ + M³⁺ recovers the emission of DNSA-SQ (Fig. S7, ESI†), showing that the binding of M³⁺ to DNSA-SQ is a reversible process.

Fig. 4 Absorption (a) and fluorescence (b) spectra of DNSA-SQ (5 μM) in CH₃CN solutions with different metal ions (20 equivalents in (a) and 2 equivalents in (b)).

Fig. 5 Relative fluorescence intensities (F₆₄₀/F₆₈₄) of DNSA-SQ (5 μM) in the presence of 2 equivalents of various metal ions (Al³⁺ is derived from AlCl₃, λ_ex = 620 nm).

Scheme 2 A plausible 2 : 1 binding mode between DNSA-SQ and a trivalent ion.

The discrimination of different trivalent cations was further investigated by increasing the water content (Fig. S8, ESI†). The ratiometric fluorescence for Al³⁺ remained unchanged when the assay solution contained 5% water. However, the fluorescence response of DNSA-SQ with Fe³⁺ showed quenching and no obvious fluorescence change was detected for Cr³⁺ under identical conditions.

Switching the selectivity to Ag⁺ in the presence of BSA

We recently found that DNSA-SQ can selectively bind to Site I of bovine serum albumin (BSA) through hydrophobic interactions, which increased its fluorescence intensity.¹⁷ To check the effect of BSA on the binding of DNSA-SQ to metal ions, the fluorescence responses of DNSA-SQ with metal ions were investigated in the presence of a certain amount of BSA. As discussed above, DNSA-SQ shows no response to monovalent Ag⁺ in an aqueous solution (Fig. S9 and 10, ESI†). However, the addition of Ag⁺ to an aqueous solution of DNSA-SQ in the presence of BSA shows an obvious fluorescence intensity increase (Fig. 6 and 7). These results indicate that the complex formed between DNSA-SQ and BSA affords additional binding sites for Ag⁺, which is beneficial for stabilizing DNSA-SQ–Ag⁺–BSA. Apart from Zn²⁺ and Al³⁺, which induced a fluorescence intensity increase to some intent, no obvious interferences by other metal ions were observed under identical conditions (Fig. 8, S12 and 13†). It is obvious that DNSA-SQ shows a different fluorescence response to various metal ions in the absence and presence of BSA. Additional binding sites on the cavity of the protein assist the coordination of the sensor to metal ions.¹⁸ The effect of different ratios between BSA and DNSA-SQ on the fluorescence response to Ag⁺ ions was investigated. When the ratio was fixed at 1 : 1, a maximum fluorescence intensity increase with Ag⁺ was observed, illustrating that a 1 : 1 ratio of BSA to DNSA-SQ was the optimal sensing parameter for Ag⁺ detection (Fig. 9). Moreover, the sensing response of DNSA-SQ to Ag⁺ experienced no observed interference from the presence of other metal ions. Thus, assisted by BSA, DNSA-SQ shows a selective response to Ag⁺. It is proposed that the biothiol moieties in BSA and the sulfur atoms on the dye form strong S–Ag bonds to fix Ag⁺.¹⁹ The synergic binding with Ag⁺ further increases the molecular rigidity of the dye, reducing the non-radiative decay of the excited state and leading to a fluorescence enhancement. Additionally, the size of the hydrophobic cavity where DNSA-SQ enters is suitable to accommodate Ag⁺. The 1 : 1 stoichiometry and the association constant (4.6 × 10³ M⁻¹) were estimated using the Benesi–Hildebrand method with the fluorescence titration data (Fig. S11, ESI†).¹³ A linear relationship between the emission intensity of DNSA-SQ at 675 nm and the Ag⁺ concentration in the range of 15–50 μM was observed.

Fig. 6 The fluorescence intensity of the probe DNSA-SQ (5 μM) at 675 nm in the presence of 1 equivalent of BSA was linearly related to the concentration of Ag⁺ (15–50 μM).

Fig. 7 Fluorescence spectra changes of DNSA-SQ (5 μM) in aqueous solutions with different concentrations of Ag⁺ in the presence of 1 equivalent of BSA.

Fig. 8 Relative fluorescence intensity change, (I₆₇₅ − I₀)/I₀, of DNSA-SQ (5 μM) in an aqueous solution upon the addition of 20 equivalents of various metal ions in the presence of 1 equivalent of BSA, where I₀ and I₆₇₅ indicate the fluorescence intensity of DNSA-SQ at 675 nm in the absence and presence of 20 equivalents of various metal ions, respectively.

Fig. 9 Relative fluorescence intensity (I₆₇₅/I₀) response of DNSA-SQ (5 μM) at 675 nm in phosphate buffer solution (10 mM, pH 7.2) with different concentrations of Ag⁺ in the presence of BSA.

The fluorescence circuits of DNSA-SQ to construct logic devices using BSA and Ag⁺ as inputs

Recently, molecular logic systems for the construction of molecular devices has gained much attention.²⁰ The important electronic logic device mimicked at the molecular level is a keypad lock, which represents a new approach for protecting information at the molecular scale and authorizing password entry to a limited number of people. Its output signal depends on both the inputs of the circuit and its previous state by which these inputs are introduced. These molecular devices can distinguish between different chemical sequences and have advantages over simple molecular logic gates. Like other molecular keypad locks, the fluorescent sensor reported here could mimic the function of a security keypad lock on the sequential addition of two substrates. To gain a molecular keypad lock system based on Boolean arithmetic from the fluorescence behavior of DNSA-SQ, BSA and Ag⁺ were used as inputs. To simplify the input sequence as a password for the molecular keypad lock, the inputs BSA and Ag⁺ ions were signed as “A” and “T”. The fluorescence intensity of DNSA-SQ at 675 nm as the output, has “On” (>100) and “Off” states (<100), where the letters “M” and “F” define the two states, respectively. When the input “A” or “T” was separately added, the emission at 675 nm was in the “Off” state (Fig. 10 and S14†). In contrast, when the input “A” was added first and followed by “T”, the emission at 675 nm was in the “On” state and it created the secret password “ATM” (Fig. 11), suggesting the operation of an AND logic gate. Reversal of the input sequence, i.e. “T” as the first input and “A” as the second input, gave a weak fluorescence response. The wrong password “TAB” fails to open the lock (Fig. 11). Therefore, only the exact password “ATM” can open the lock, and such types of fluorescence systems may be applicable to protecting information at the molecular scale as they require a specific combination and the correct order of inputs. The wrong password, “TAB”, is probably ascribed to the fact that the Ag–S linking, derived from the biothiol moieties in BSA and Ag, blocks DNSA-SQ from accessing the hydrophobic cavity of BSA.

Fig. 10 Change in fluorescence intensity of DNSA-SQ (5 μM) upon input of BSA and Ag⁺ in buffer solution (10 mM PBS, pH = 7.4). The truth table indicates AND logic functions.

Fig. 11 Molecular keypad lock with a secret code of fluorescence output at 675 nm.

Conclusions

In summary, we have presented a simple sensor, DNSA-SQ, possessing ICT properties for ratiometric fluorescence responses to trivalent ions Al³⁺, Fe³⁺ and Cr³⁺. The ratiometric fluorescence changes are ascribed to 2 : 1 complexes between DNSA-SQ and the metal ions. Furthermore, DNSA-SQ can switch its fluorescence response to Ag⁺ in the presence of BSA, which provides additional binding sites and a perfect microenvironment for Ag⁺ sensing. By judiciously mediating the sensing system, fluorescent sensors for trivalent ions or Ag⁺ ions have been established. In addition, a molecular keypad lock based on the fluorescence “turn-on” output at 675 nm with sequential chemical inputs of BSA and Ag⁺ ions has been made using a synergistic effect. Utilization of fluorescent assembles of squaraine dyes in the design and construction of functional security devices is promising.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Grant no. 21206137) and the Scientific Research Foundation of Northwest A&F University (Z111021103 and Z111021107).

Notes and references

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Footnote

† Electronic Supplementary Information (ESI) available: Additional UV-vis and fluorescence spectra. See DOI: 10.1039/c3ra47635a

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