Eco-friendly Fluorescent Sensor for Sensitive and Selective Detection of Zn²⁺ and Fe³⁺ Ions: Applications in Human Hair Samples

Abstract

A new eco-friendly sensor, 3-((6-((4-chlorobenzylidene)amino)pyridin-2-yl)imino)indolin-2-one (CBAPI) was synthesized and well characterized. The CBAPI sensor was employed for detecting Zn²⁺ and Fe³⁺ ions. It exhibited a low limit of detection at pH 6.0, with values of 2.90, for Zn²⁺ and 3.59 nmol L⁻¹ for Fe³⁺ ions. The sensor demonstrated high selectivity over other interfering cations. Additionally, the high binding constants reflect the great affinity of sensor towards Zn²⁺ and Fe³⁺ ions. To further validate its quantification ability for Zn²⁺ ions, the synthesized CBAPI sensor was used to determine Zn levels in human hair samples, and the results were confirmed using atomic absorption spectroscopy (AAS). The AGREE metric tool was used to assess the method’s environmental impact and practical applicability. These positive outcomes indicated that the new method for detecting Zn²⁺ and Fe³⁺ ions is environmentally friendly and safe for humans. The developed CBAPI sensor represents a potential development in metal ion detection, combining sensitivity, selectivity, and rapidity.

Supplementary Information

The online version contains supplementary material available at 10.1007/s10895-024-03798-3.

Introduction

Industrial activities over the past century have significantly increased human exposure to heavy metals, whether through water, air, or food. This exposure can lead to acute or chronic poisoning. Heavy metals accumulate in the body, causing various toxic effects on tissues and organs. Additionally, they interfere with cellular processes such as growth, differentiation, and apoptosis [1].

Several heavy metals lack any known physiological benefits for humans. Lead, mercury, and cadmium are prime examples of such toxic metals. However, other metals are essential to human biochemical processes. For instance, zinc and iron serve as important cofactors for several enzymatic reactions in the human body [2].

Zinc, a trace element found in both human and natural environments, serves crucial roles in different biological processes. It is essential for normal growth and reproduction in higher plants, animals, and humans. Additionally, it contributes significantly to physiological growth and fulfils immune functions. Zinc is vital for the functionality of more than 300 enzymes, DNA stabilization, and gene expression [3, 4]. Similarly, iron is an essential element for nearly all living organisms. Due to its ability to exist in one of two oxidation states, it plays a vital role in diverse metabolic processes like oxygen transport, DNA synthesis, and electron transport. However, maintaining precise iron levels in body tissues is crucial because excessive iron can cause tissue damage by promoting the formation of free radicals [5, 6].

Various methods are employed for detecting Zn²⁺ and Fe³⁺ ions, including atomic absorption spectroscopy, inductively coupled plasma (ICP), and electrochemical techniques [7–20]. While these methods offer high sensitivity and selectivity, they often require costly equipment, complicated procedures, and time-intensive processes. In contrast, optical sensors provide valuable tools for precisely identifying clinical, chemical, and environmental species [21, 22]. Fluorescent sensors offer rapid and sensitive characterization of different species in samples. This technique stands out for its high sensitivity, specificity, simplicity, and cost-effectiveness compared to other analytical tools. It finds widespread use in environmental monitoring, industrial processes, medical diagnostics, forensics, and genetic analysis. Fluorescent sensors are applicable for both quantitative and qualitative analysis [23].

Our ongoing research focuses on creating chemosensors capable of detecting a range of environmentally and biologically relevant species [24–34]. In this study, we synthesized a novel fluorescent sensor, 3-((6-((4-chlorobenzylidene) amino) pyridin-2-yl)imino)indolin-2-one (CBAPI). This sensor has been successfully applied for the selective and sensitive detection of Zn²⁺ and Fe³⁺ ions.

Experiments

Materials and Reagents

Zinc nitrate Zn(NO₃)₂.6H₂O, Ferric nitrate Fe(NO₃)₃.9H₂O, and organic solvents were provided by Sigma Aldrich used as analytical grade with purity of ≥ 99.0%. all cationic compounds salts of Ag⁺, Al³⁺, Cu²⁺, Co²⁺, Hg²⁺, Ni²⁺, and Pd²⁺ were purchased from Aldrich.

Instruments

Utilizing A Shimadzu-UV Probe Version 2.33 spectrophotometer, and JASCO FP-8300 Fluorescence Spectrophotometer.

Methods

Solution Preparations

By dissolving 0.0336 g of CBAPI sensor in a 100 mL DMF to get a 1.0 × 10⁻³ mol L⁻¹ stock solution. 0.0297 g and 0.0404 g of Zn(NO₃)₂.6H₂O and Fe(NO₃)₃.9H₂O, respectively, were dissolved in 100 mL bi-distilled water get a 1.0 × 10⁻³ mol L⁻¹ stock solutions. The working solutions were prepared by diluting stock solutions to a known volume with bi-distilled water. Buffer solutions with pH values of 2.0, 4.0, 6.0, 8.0, and 10.0 were used.

Synthesis of 3-((6-aminopyridin-2-yl)Imino)Indolin-2-one (3)

To a solution of 2,6-diaminopyridine (1) (0.005 mol) in ethanol (20 mL), isatin3(2) (0.005 mol) was added. The reaction mixture was heated under reflux for 2 h, the solid compound (3) was filtered after cooling, washed with ethanol and used directly in the next reaction.

Synthesis of 3-((6-((4-Chlorobenzylidene)amino)Pyridin-2-yl)Imino)Indolin-2-one (4) (CBAPI)

A mixture of 3-((6-aminopyridin-2-yl)imino)indolin-2-one (3) (0.005 mol) and 4-chlorobenzaldehyde (0.005 mol) in butanol (20 mL) was heated under reflux for 4 h, the reaction was then cooled, the solid product obtained after cooling was collected by filtration and recrystallized from butanol to give (4) as red crystals. (yield 72%, m.p. over 300 °C. FTIR (ATR, Ʋ, cm⁻¹): 3365 (NH), 1720 (CO), 1608 (C = N). ¹H NMR (500 MHz, DMSO-d₆) δ 10.13 (s, 1H), 9.96 (s, 1H), 7.93–7.87 (m, 2H), 7.57–7.50 (m, 2H), 7.46 (d, J = 7.1 Hz, 2H), 7.05–6.99 (m, 3H), 6.87 (d, J = 7.8 Hz, 2H).

Investigating the Impact of Zinc and Iron on CBAPI Fluorescence

0.2 mL of CBAPI sensor (2.0 × 10^–5 mol L⁻¹) solution and 4 mL of buffer solution of pH 6.0 were transferred to a 1 cm quartz cell, and successive additions of Zn²⁺ (0.00– 9.09 × 10^–5 mol L⁻¹ ppm) solution or Fe³⁺ (0.00– 3.54 × 10^–5 mol L⁻¹ ppm) solution were added. Within a few seconds of the addition of different analyte concentrations (Zn²⁺ or Fe³⁺ ions), the emission intensity measurements were monitored, and a calibration curve was developed, revealing an emission signal assigned at λ_em = 408 nm after applying excitation at λ_ex 335 nm.

Assaying Zinc in Human Hair Samples

One gram of hair was cut using scissor to pieces of less than 1 cm, then washed using bi-distilled water, then methanol, and left to dry in the air at room temperature. 12 mL of 90% nitric acid was added to dry hair and kept at room temperature for 10 min, then 1 ml of concentrated perchloric acid was added and heated on a hot plate at 200 °C for 1 h. until brown fumes converted into dense white fumes [35]. The clear solution was allowed to cool and dilute to 25 mL with bi-distilled water, and then analyzed using atomic absorption spectroscopy.

Results and Discussion

Characterization of CBAPI Sensor

The starting 3-((6-aminopyridin-2-yl) imino) indolin-2-one (3), was synthesized by the condensation reaction of 2,6-diaminopyridine (1) with isatin (2). Subsequent condensation of (3) with 4-chlorobenzaldehyde afforded the target Schiff base (4) (Scheme 1).

Scheme 1.

The synthesis mechanism of CBAPI sensor

The proposed structure of (4)CBAPI sensor was confirmed from spectral data. The IR spectrum showed absorption band at 3365 cm⁻¹ which may be assigned due to ν(N–H) stretching of the indoline ring. A strong absorption band was observed at 1720 cm⁻¹ indicating the presence of ν(C = O) group of isatin. The IR spectrum also revealed the presence of absorption band corresponding to the ν(C = N) at 1608 cm⁻¹ (SI: Fig. 1).

The ¹H-NMR spectrum of compound (4) showed a broad singlet at 10.13 ppm which may be assigned due to proton of the NH group of indoline ring. The signal assigned to the azomethine proton was observed at 9.96 ppm as singlet signal. Because of the many J values and overlapping signals, the signals of the aromatic protons were observed as complex multiplets at around δ 6.87–7.93 ppm (SI: Fig. 2).

Effect of Solvent on Absorption and Emission Spectrum of the CBAPI Sensor

Figure 1 illustrates the UV − vis spectra of 1 × 10^–4 mol L⁻¹CBAPI sensor in different solvents (ethanol, methanol, water, DMF, isopropyl alcohol, benzene, methylene chloride, cyclohexane, and toluene) (1:99) (v/v) DMF: solvent and the corresponding spectroscopic parameters are collected in Table 1. The emission spectrum of CBAPI sensor in different solvents at λ_exc 335 nm was depicted in Fig. 2.

Fig. 1.

UV absorption spectra for 1 × 10^–4 mol L⁻¹CBAPI sensor in different solvents

Table 1.

Spectral and photophysical parameters of CBAPI sensor in different solvents

Solvent	λabs (nm)	λexc (nm)	λem (nm)	ε (mol−1 cm−1 L) × 103	Stock shift (cm−1) Δʋ = ʋexc—ʋem
Methanol	315, 501	335	411	9.27, 1.78	5519.8
Ethanol	316, 501	335	416	9.10, 1.72	5812.3
Water	321, 517	335	402	7.91, 1.41	4975.1
Isopropyl	321, 506	335	394	8.96, 1.77	4470.0
DMF	322, 507	335	417	8.77, 1.69	5869.9
Toluene	314, 501	335	371	7.01, 1.97	2896.6
Methylene Chloride	315, 463	335	396	9.16, 1.81	4598.2
Benzene	319, 511	335	375	7.20, 1.36	3184.1

Fig. 2.

Emission spectra for 2 × 10^–5 mol L⁻¹CBAPI sensor at different solvents with λ_exc 335 nm

The absorption spectra of the CBAPI sensor exhibited negligible changes with changing solvent polarity. Two absorption peaks occur at approximately 320 nm and 500 nm. These peaks correspond to π → π* and n → π* transitions, respectively. The molar absorption coefficients for these transitions fall within the range of 1360–9270 mol⁻¹ L cm⁻¹, varying based on the solvent used. The peak assigned to the n → π* transition was affected by a large red shift recorded in water over other solvents used (λ_abs 517 nm), this might be due to the formation of a strong hydrogen bond between the CBAPI sensor and the water molecule. It is clearly shown that the absorption maximum of the CBAPI sensor shows nearly no noticeable shift with an increase in polarity of the solvent, but the emission maximum exhibited an appreciable bathochromic shift with an increase in solvent polarities from 390 to 415 nm on exciting at 335 nm, indicating nearly zero dipole moments in the ground state but large dipole moments in the excited state.

Figure 3 shows the absorption spectra of the CBAPI sensor at different pH values (pH 2 to pH 10) using different buffer solutions. It was noticed that with decreasing pH, the absorption peak assigned to π → π* is greatly shifted towards longer wavelengths (red shift) from λ = 320 nm for pH 10.0 to λ = 340 nm for pH 2.0, while the absorption peak assigned to n → π* is slightly affected by changing pH values. The π-π* peak is indeed pH-dependent, while the n-π* peak shows only minor sensitivity. In an acidic medium, the carbonyl oxygen of the isatin moiety is protonated, affecting the electron density distribution of the CBAPI molecule, and causing changes in the energy levels of the π and π* orbitals. Consequently, the π-π* peak position is altered due to this protonation [36–38].

Fig. 3.

Absorption Spectra of 1 × 10^–4 mol L.⁻¹CBAPI sensor at different pH values (pH 2 – pH 10) in DMF/water (0.2:9.8, v/v)

Optimizing the Emission Intensity of the CBAPI Sensor for Zn²⁺and Fe³⁺Ions Detection

pH Influence

“The impact of varying pH values on the detection of Zn (II) or Fe (III) using the CBAPI sensor was investigated across a wide range of pH values (from pH 2 to pH 10) using different buffer solutions (Fig. 4). It was noticed that the higher emission intensity for both metals under study was recorded at pH 6, so further studies were conducted at pH 6 using acetate buffer, which is within an appropriate range for biological and environmental applications. The low intensity at low pH values (pH ≤ 4) and higher pH values (pH ≥ 8) may be due to protonation of imine nitrogen N and carbonyl oxygen C = O of the isatin moiety and precipitation of metal hydroxide, respectively.

Fig. 4.

Emission intensity at λ_em = 408 nm for detection of Zn (II) and Fe (III) ions as a function of pH using the CBAPI sensor

Method Validation

Linearity and Sensitivity

The fluorescence titration of different concentrations of metal ions under study (Zn²⁺ or Fe³⁺) to a 20 µmol L⁻¹ CBAPI sensor was carried out at pH 6.0 using acetate buffer and at λ_exc = 335 nm. Upon the addition of Zn²⁺ ions, there was a notable red shift from the peak at 410 nm to 422 nm, with a remarkable enhancement of emission intensity. Upon the addition of Fe³⁺ ions, a remarkable enhancement of emission intensity is only detected without any shift in wavelength (Fig. 5).

Fig. 5.

The emission spectra of the CBAPI sensor at λ_exc 335 nm in acetate buffer (pH = 6.0) with increasing the concentration of Zn²⁺ and Fe³⁺ ions from 3.3 nmol L⁻¹ to 80 µmol L.⁻¹

The calibration curve depicted in Fig. 6 showed a linear enhancement in intensity with increasing Zn²⁺ or Fe³⁺ concentrations. The results of the calibration curve for Zn²⁺ determination exhibited two linear ranges: 3.3 to 70 nmol L⁻¹ and 3.3 to 80 µmol L⁻¹. In the first range, the equation for linear regression is F = 1197.6 + 3.47 × 10⁹ [Zn²⁺] accompanied by a high correlation coefficient of 0.9972, whereas in the subsequent linear range, the linear regression equation is F = 1449.2 + 2.49 × 10⁶ [Zn²⁺] with a correlation coefficient of 0.9888. While the results of the calibration curve for Fe³⁺ determination exhibited two linear ranges: 3.3 to 90 nmol L⁻¹ and 0.33 to 35 µmol L⁻¹. In the first range, the equation for linear regression is F = 1451.75 + 1082 × 10⁹ [Fe³⁺] with a correlation coefficient of 0.9861, whereas in the second linear range, the linear regression equation is F = 1675.27 + 1.42 × 10⁷ [Fe³⁺] with a correlation coefficient of 0.9591. The CBAPI sensor exhibits greater sensitivity to Zn²⁺ or Fe³⁺ ions, as evidenced by the steeper slope observed in the first linear range compared to the second. The sensitivity of the measurement decreases as the concentration of Zn²⁺ or Fe³⁺ ions increase due to the binding sites of the CBAPI sensor being occupied.

Fig. 6.

The relationship between the fluorescence intensity of the CBAPI sensor and the Zn²⁺ or Fe.³⁺ ions concentration at λ_exc 335 nm, λ_em 408 nm in acetate buffer (pH = 6.0)

The detection limits (D_L) (3SD/slope) and quantification limits (Q_L) (10SD/slope) were calculated and presented in Table 2. The detection limits for Zn²⁺ or Fe³⁺ ions were 2.90 and 3.59 nmol L⁻¹, respectively, and the quantification limits for Zn²⁺ or Fe³⁺ ions were 9.68 and 11.98 nmol L⁻¹, respectively. The detection limits for Zn²⁺ or Fe³⁺ ions are compared to those of previously reported sensors (Table 3), and the results provide low detection limits over many fluorescent sensors in the literature [39–67]. Furthermore, the World Health Organization (WHO) has defined safe levels of zinc or ferric ions in drinking water as 4.58 × 10⁻⁵ and 5.37 × 10⁻⁵ mol L⁻¹, which significantly exceed the levels calculated by the CBAPI sensor [19, 68].

Table 2.

The correlation coefficients, D_L (detection limit), Q_L (quantitation limit), and K_D (binding constant) for determination of Zn²⁺ or Fe³⁺ ions using the CBAPI sensor at λ_em 408 nm, λ_exc 335 nm and at pH 6.0

Parameter	Zn (II) ion	Fe (III) ion
DL (nmol L−1)	2.90	3.59
QL (μmol L−1)	9.68	11.98
Regression coefficient	0.9972	0.9861
Slope	3.47 × 109	1.82 × 109
Intercept	1.20 × 103	1.45 × 103
Standard deviation	3.36 (n = 24)	2.18 (n = 31)
Linearity range (nM)	3.3—70	3.3—90
Binding Constant KD (M−1)	1.77 × 107	6.41 × 106

Table 3.

Comparison between the CBAPI sensor with the literature-reported methods for Zn²⁺ and Fe³⁺ ions detection

Sensor	Detection limit (nmol L−1)	Reference
Detection limits of Zn (II) ions
N,N′-phenylenebis(salicylideaminato)	1.5 × 102 nM	[39]
2-(benzo[d]thiazol-2-yl) phenol (BTP)	23.6 nM	[40]
2-(((pyridin-2-ylmethyl)imino)methyl)phenol	62 nM	[41]
di-2-picolylamine-dithiolcarbamate (DPA-DTC)/proline-dithiolcarbamate (P-DTC)	700 nM	[42]
Fluorescent quantum dots (QDs) as a promising alternative for organic dyes CdTe QDs	1.2 × 103 nM	[43]
3-(benzo[d]thiazol-2-yl)-4-hydroxy-2H-chromen-2-one	35.8 nM	[44]
(E)-N’-(5-allyl-2-hydroxy-3-methoxybenzylidene) nicotinohydrazide	4.35 nM	[45]
tert-butyl (2-bromoethyl)carbamate (4)	107 nM	[46]
tetraphenylporphyrin β-cyclodextrin	5.0 × 102 nM	[47]
The Schiff-base ligand	95.3 nM	[48]
TYMN ((E)-1-((thiazol-2-ylimino)methyl)naphthalen-2-ol)	31.1 nM	[49]
(L, Dansyl-Asp-His-NH2)	36.8 nM	[50]
1-[[(2-furanylmethyl)imino]methyl]-2-hydroxyjulolidine	7.50 × 103 nM	[51]
Methyl 2-((7-hydroxy-4-methyl-2-oxo-2H-chromen-8-yl)methyleneamino)-3-phenyl- propanoate	3.43 nM	[52]
2,20 -(1E,10 E)-(2,2-azanediylbis(ethane2,1-diyl)bis(azan -1-yl-1-ylidene))bis(methan-1-yl-1-ylidene) dipnenol, TAS	83 nM	[53]
CBAPI sensor	2.90 nM	Present study
Detection limits of Fe (III) ions
2-azido-1-ethanol, 910-bis(bromo- methyl)anthracene, 2-(prop-2-yn-1-yloxy)naphthalene	3.14 × 102 nM	[54]
Rhodamine-based probe (RC)	14 nM	[55]
Fluorescent probe Rh-2 based on bis- (rhodamine)	12.4 nM	[56]
Polyethyleneimine (PEI)–modified reduced graphene oxide (rGO)	1.12 × 103 nM	[57]
Porous copper nanoclusters (p-Cu NCs)	23.4 nM	[58]
((E)-2-((4-(diethylamino)benzylidene)amino)benzoic acid, DBAB)	21.7 nM	[59]
Fluorescein isothiocyanate (FITC)	4.60 × 103 nM	[60]
Carbon quantum dots (CQDs)	1.37 × 104 nM	[61]
Fluorescent carbon nanoparticles (CNPs) by using dopamine	3.20 × 102 nM	[62]
Dual-emission carbon dots DCDs	8.0 × 102 nM	[63]
Dual-response fluorescent probe (QLBM) based on quinoline and benzimidazole groups	1.24 × 102 nM	[64]
Water soluble carbon nanoparticles (CNPs)	3.21 × 105 nM	[65]
Fluorescence resonance energy transfer (FRET)	2.54 × 103 nM	[66]
4,4′-(thiazolo[5,4-d]thiazole-2,5-diyl)bis(N,N-bis(4-methoxyphenyl)aniline) (TTz-1)	0.29 × 102 nM	[67]
CBAPI sensor	3.59 nM	Present study

Method Selectivity

The selectivity of the CBAPI sensor to other metal ions was separately investigated under optimum conditions. The emission spectrum of the 20 µmol L⁻¹CBAPI sensor in DMF/water (0.2:9.8, v/v) at pH 6.0 and λ_exc 335 nm was monitored in the presence of 20 µmol L⁻¹ of interfering ions (Ag⁺, Al³⁺, Cu²⁺, Co²⁺, Hg²⁺, Ni²⁺, and Pd²⁺) (Fig. 7), and the emission intensity at 408 nm was recorded (Fig. 8). The results showed that all cations displayed an insignificant change in the emission spectra of the CBAPI sensor.

Fig. 7.

The emission spectra of the CBAPI sensor at pH = 6.0 in the presence of different metal ions at λ_exc 335 nm

Fig. 8.

Intensity at λ_em 408 nm of CBAPI sensor in presence of 10 μmol L⁻¹ different metal ions at λ_ex 335 nm and pH 6.0

Molar Ratios and Binding Constants

To further demonstrate the stoichiometry between the CBAPI sensor and Zn (II) or Fe (III) ions, the molar ratio method was used, as shown in SI: Fig. 3. Emission intensities at λ_em = 408 nm and at pH 6.0 are measured when using different concentrations of the CBAPI sensor (from 1 × 10^–5 mol L⁻¹ to 4.5 × 10^–5 mol L⁻¹), while the concentration of metal ions under study is kept constant (1 × 10^–5 mol L⁻¹). The [CBAPI]/[M] ratio was simultaneously plotted against the fluorometric intensity for each complex, the results showed 2:1 and 3:1 (CBAPI-to-metal) ratios for Zn/CBAPI complex and Fe/CBAPI complex, respectively.

The Benesi-Hildebrand equation (Eq. 1) was used to determine the binding constants (K_D) between the CBAPI sensor and the metal ions (Zn (II) or Fe (III)) under investigation [69]. K_D (unit of liter per mole) is derived from the quotient of y-intercept and slope. Where F_L is the limiting intensity of fluorescence, and [Mⁿ⁺] is the concentration of Zn (II) or Fe (III) ions. F^o and F represent the emission intensity at λ_em = 408 nm and at pH 6.0 in the presence and absence of metal ions under study, respectively.

$$\frac{F^{\circ }}{F-F^{\circ }}=\propto +\frac{\propto }{K_D\left[M^{n+}\right]},\propto =\frac{1}{F_L-F^{\circ }}$$ 1

A plot of F^o/(F-F^o) versus 1/[Mⁿ⁺] gave a linear relationship (SI: Fig. 4) where the concentration of the CBAPI sensor was kept constant (2 × 10^–5 mol L⁻¹), while the concentration of metal ions under study (Zn (II) or Fe (III)) set up from 3.3 nmol L⁻¹ to 80 µmol L⁻¹. Using this calculation method, the binding constants K_D for Zn/CBAPI and Fe/CBAPI complexes were determined to be 1.77 × 10⁷ and 6.41 × 10⁶ M⁻¹ at pH 6.0.

Mechanism of Fluorescence Sensitization

The binding mechanism involves the coordination of Zn²⁺ or Fe³⁺ ions with the imine nitrogen (azomethine) and carbonyl oxygen of the ketone group in the isatin moiety. This coordination leads to a remarkable enhancement of the emission intensity at 408 nm in the CBAPI sensor, making it valuable for analytical and biological applications in detecting Zn²⁺ or Fe³⁺ ions (SI: Scheme 1). Additionally, the chelation to Zn²⁺ or Fe³⁺ ions induces rigidity in the formed complexes, thereby increasing the Chelation Enhanced Fluorescence (CHEF) effect [70].

Application to Human Hair Samples

To further confirm the quantification ability of the CBAPI sensor to detect Zn²⁺ ions, The synthesized (2.0 × 10^–5 mol L⁻¹) CBAPI sensor was applied to determine the concentrations of Zn in two human hair samples at pH 6.0. The extracted Zn²⁺ ions were analyzed using Perkin Elmer Analyst 100 Atomic Absorption Spectroscopy (AAS), and the results were compared with the results obtained when using the CBAPI sensor. As shown in Table 4, the results of two hair samples were in good agreement with AAS results, which shows that the CBAPI sensor is an effective and accurate sensor for detecting Zn in human hair elements.

Table 4.

Concentrations (ppm) of Zn²⁺ ions in human hair

Sample	Concentration of proposed method (ppm)
	AAS	CBAPI sensor
Sample No. 1	0.355	0.348
Sample No. 2	0.367	0.363

Assessing Analytical GREEnness Metric Using CBAPI Sensor

The reliability of the greenness attributes of the spectrofluorimetric method for detecting Zn²⁺ or Fe³⁺ ions using the CBAPI sensor was evaluated through the Evaluation of Greenness Attributes [71, 72]. The synthesized sensor received an AGREE score of 0.85, reflecting the high greenness attributes of the fluorometric detection of Zn²⁺ or Fe³⁺ ions based on the CBAPI sensor. This high score (0.85) is primarily attributed to the detection method carried out in DMF/water (0.2:9.8, v/v) at a pH near the physiological value (pH 6.0), without the use of toxic solvents even during the extraction and preconcentration procedures emphasizing the eco-friendly nature of this approach (Fig. 9).

Fig. 9.

AGREE Assessment for detecting of Zn²⁺ or Fe³⁺ ions using CBAPI sensor

Conclusion

The CBAPI sensor was synthesized and characterized using FT-IR and ¹H-NMR spectroscopy. Its spectroscopic parameters in different solvents were determined. The CBAPI sensor, with its remarkable sensitivity and selectivity, has emerged as a valuable tool for detecting Zn²⁺ and Fe³⁺ ions, and its performance was compared with the AAS technique for zinc detection in human hair samples. By calculating detection and quantification limits through fluorescence titration under optimum conditions, we demonstrated the strong binding affinity between Zn²⁺ or Fe³⁺ ions and the CBAPI sensor, as well as its high selectivity over other interfering cations. Finally, the proposed method achieved a high AGREE score, emphasizing its eco-friendly nature.

Supplementary Information

Below is the link to the electronic supplementary material.

Funding

Open access funding provided by The Science, Technology & Innovation Funding Authority (STDF) in cooperation with The Egyptian Knowledge Bank (EKB). No funds, grants, or other support were received.

Data Availability

Not applicable.

Declarations

Ethical Approval

Not applicable.

Competing Interest

The authors have no conflicts of interest to declare that are relevant to the content of this article.