Development and application of a Schiff-base colorimetric sensor for lead (Pb²⁺) detection in borehole water from Kaduna metropolis, Nigeria

Abstract

Lead contamination of groundwater remains a serious environmental and public health concern, particularly in urban regions where borehole water serves as a major domestic water source. Conventional analytical techniques such as atomic absorption spectroscopy (AAS) and inductively coupled plasma-based methods provide excellent sensitivity for trace metal determination but are often limited by high cost, technical expertise requirements, and infrastructural constraints in many developing regions. In this study, a simple Schiff-base ligand derived from salicylaldehyde (0.01 mol) and ethylenediamine (0.01 mol) was synthesized under reflux for 2 h and evaluated as a low-cost colorimetric sensor for Pb²⁺ detection in borehole water collected from Kaduna metropolis, Nigeria. The ligand was obtained as a yellow crystalline product with a percentage yield of 78.4% and melting point of 189–191 °C. FT-IR analysis confirmed successful azomethine formation with ν(C=N) band at 1624 cm⁻¹, while 1H NMR spectroscopy showed characteristic azomethine proton resonance at δ 8.31 ppm, aromatic proton signals at δ 6.82–7.45 ppm, methylene linker protons at δ 3.76 ppm, and phenolic OH resonance at δ 12.94 ppm, confirming ligand formation. Coordination of Pb²⁺ through azomethine nitrogen and phenolic oxygen donor sites was accompanied by a shift of the ν(C=N) band to 1598 cm⁻¹ and formation of a probable distorted hemidirected Pb(II) coordination geometry, producing a visible color change from pale-yellow to orange with the emergence of an absorption band at 445 nm. Good linearity was obtained over the investigated concentration range with correlation coefficient (R² = 0.995), slope-based sensitivity of 0.126 AU mg⁻¹ L, limit of detection (LOD) of 0.82 mg/L, and limit of quantification (LOQ) of 2.49 mg/L. Selectivity studies showed stronger response toward Pb²⁺ relative to competing ions such as Cu²⁺, Zn²⁺, Ni²⁺, Fe³⁺, and Cd²⁺ tested at 50 mg/L under identical conditions. Application to twelve borehole water samples produced recovery values of 92–98% with relative standard deviations below 3.0%, indicating satisfactory precision and analytical applicability. Although not intended to replace ultra-trace instrumental methods, the developed sensor offers a simple, rapid, and affordable approach for preliminary groundwater lead monitoring and may serve as an early-warning screening tool for potentially contaminated water sources in resource-limited settings.

Introduction

Groundwater remains one of the most important sources of freshwater for domestic, agricultural, and industrial use worldwide. In many developing countries, including Nigeria, borehole water is extensively relied upon for drinking and household purposes because of inadequate municipal water supply systems, rapid urban population growth, and increasing pressure on public water infrastructure (Adelana & MacDonald, 2008). In numerous urban centers, privately owned and community boreholes provide the most accessible and affordable source of water for households, schools, healthcare facilities, and small businesses. However, increasing dependence on untreated groundwater has raised significant concerns regarding water quality and public health, particularly in areas affected by industrialization, poor sanitation, indiscriminate waste disposal, and geogenic contamination pathways (Alloway, 2013).

Among the various pollutants that threaten groundwater quality, heavy metals are of particular concern because of their persistence, non-biodegradable nature, tendency to bioaccumulate, and long-term toxicological effects (Dwanga et al., 2024). Unlike many organic contaminants, heavy metals do not readily decompose in the environment and may remain in soils, sediments, and water bodies for prolonged periods. Their transport and mobility in aquifer systems are influenced by pH, redox potential, mineralogy, and anthropogenic disturbances, thereby increasing the risk of human exposure through drinking water sources (Alloway, 2013).

Lead (Pb²⁺) is one of the most hazardous environmental heavy metals because it has no known beneficial biological role in humans and may exert harmful effects even at relatively low concentrations. Chronic lead exposure has been associated with neurological impairment, reduced cognitive development in children, kidney dysfunction, hypertension, anemia, reproductive toxicity, and cardiovascular disorders (Flora et al., 2012). Children, infants, and pregnant women are especially vulnerable because lead may interfere with nervous system development and cross the placental barrier. Owing to these risks, the World Health Organization recommends a maximum permissible concentration of 0.01 mg/L for lead in drinking water. This stringent guideline reinforces the urgent need for rapid preliminary monitoring tools capable of identifying water sources that may require confirmatory laboratory analysis (Needleman, 2004).

Lead contamination of groundwater may arise from both natural and anthropogenic sources. Weathering of lead-bearing minerals can contribute trace quantities to groundwater; however, elevated concentrations are more commonly associated with human activities such as battery recycling, metal fabrication, paint residues, plumbing corrosion, industrial discharge, mining, landfill leachates, mechanic workshops, traffic emissions, and poor waste management practices (Akan et al., 2014; Ojekunle et al., 2016). In rapidly urbanizing cities such as Kaduna metropolis, where residential settlements coexist with commercial activities, road traffic, light industries, and informal waste disposal practices, shallow groundwater systems may be particularly vulnerable to heavy-metal contamination if routine surveillance is absent.

Several studies conducted in Nigeria have reported detectable levels of toxic metals in groundwater and surface water systems, especially near industrial estates, refuse dumpsites, automobile repair zones, and densely populated settlements (Akan et al., 2014). Elevated concentrations of Pb, Cd, Cr, and Ni have been observed in some water sources, highlighting the need for continuous environmental monitoring and accessible analytical technologies capable of rapid contamination screening.

Conventional determination of lead in environmental samples is commonly performed using atomic absorption spectroscopy (AAS), inductively coupled plasma optical emission spectroscopy (ICP-OES), inductively coupled plasma mass spectrometry (ICP-MS), anodic stripping voltammetry, and X-ray fluorescence methods. These techniques are widely recognized for their sensitivity, selectivity, and analytical reliability (Skoog et al., 2018). Nevertheless, they are often associated with high capital cost, expensive maintenance, laboratory-based operation, sample preparation demands, and the need for skilled personnel. Such constraints may limit routine monitoring in many developing regions where technical infrastructure and analytical resources remain insufficient (Abdulrahman et al., 2018; Leibrand et al., 2019).

These limitations have stimulated growing interest in alternative analytical strategies that are rapid, low-cost, portable, and suitable for field deployment. Among such approaches, colorimetric sensing methods are particularly attractive because analyte detection may occur through visible color change that can be observed directly or measured spectrophotometrically with simple instrumentation (Yusuf et al., 2015). Colorimetric systems are especially useful for preliminary screening programs, emergency water testing, and resource-limited communities requiring quick decisions regarding water safety.

Schiff-base ligands constitute an important class of compounds in coordination and analytical chemistry because of their ease of synthesis, structural versatility, strong chelating ability, and tunable optical properties. They are commonly formed through condensation reactions between primary amines and aldehydes or ketones, producing azomethine (–C=N–) functionalities capable of coordinating metal ions through nitrogen donor atoms, often alongside oxygen donor sites where phenolic groups are present (Marinova & Tamahkyarova, 2025). Upon metal binding, Schiff-base ligands frequently exhibit pronounced changes in color, fluorescence, or absorbance, making them valuable candidates for chemical sensing applications.

Numerous Schiff-base-based systems have been reported for the detection of environmentally relevant metal ions such as Cu²⁺, Fe³⁺, Hg²⁺, Cd²⁺, Zn²⁺, and Pb²⁺ (Ocak et al., 2025). However, many previously reported Pb²⁺ sensors emphasize excellent laboratory sensitivity while remaining limited by complex synthetic procedures, expensive fluorometric instrumentation, unstable sensing media, or inadequate validation in real environmental samples. Consequently, practical applicability in low-resource settings is often reduced despite promising analytical performance.

There therefore remains a need for sensing systems that combine affordability, ease of synthesis, visible response, portability, and applicability to real groundwater matrices. Structurally simple Schiff-base ligands prepared from readily available reagents may provide an effective intermediate solution between expensive laboratory instrumentation and complete absence of water quality surveillance.

In the present study, a Schiff-base ligand derived from salicylaldehyde and ethylenediamine was synthesized and evaluated as a low-cost colorimetric sensor for Pb²⁺ detection in borehole water samples collected from Kaduna metropolis, Nigeria. The ligand was characterized using Fourier transform infrared (FT-IR), ultraviolet-visible (UV-Vis), and proton nuclear magnetic resonance (¹H NMR) spectroscopy, while analytical performance was assessed in terms of linearity, sensitivity, selectivity, detection limits, and recovery in real water samples.

Although the ligand framework employed in this work is structurally simple and previously known, its practical application as a rapid Pb²⁺ screening sensor for borehole water in Kaduna metropolis, supported by analytical validation and environmental relevance, constitutes the principal novelty of the present study. By combining straightforward synthesis, visible detection response, and application to real groundwater samples, the developed system may serve as an early-warning monitoring tool for communities lacking routine access to advanced analytical facilities.

Materials and methods

Study area description

The study was conducted within Kaduna metropolis, Kaduna State, Nigeria. Kaduna metropolis is located in north-central Nigeria and serves as a major administrative, industrial, and military center. The city experiences rapid urban expansion, high population density, and diverse anthropogenic activities, including textile manufacturing, metal fabrication, automobile repair workshops, small-scale industries, and extensive residential development. These activities, combined with inadequate waste management systems, present potential risks for groundwater contamination.

The geology of Kaduna State is dominated by Precambrian Basement Complex rocks, including granites, gneisses, and schists, which can influence groundwater chemistry through weathering and leaching processes. Borehole water remains a primary source of domestic water supply across the metropolis due to inconsistent municipal water provision. The selection of Kaduna metropolis for this study was informed by its heavy reliance on groundwater resources and previous reports indicating variable physicochemical quality and trace metal presence in borehole water (Adelana & MacDonald, 2008; Sadiq et al., 2020).

Chemicals and reagents

All chemicals used in this study were of analytical reagent grade and were used as received without further purification. Salicylaldehyde (≥ 99.0%, Sigma-Aldrich, Germany), ethylenediamine (≥ 99.5%, Merck, Germany), lead(II) nitrate [Pb(NO3)2] (≥ 99.0%, BDH Chemicals, UK), copper(II) nitrate trihydrate (≥ 99.0%, Sigma-Aldrich, Germany), zinc(II) nitrate hexahydrate (≥ 98.0%, Sigma-Aldrich, Germany), nickel(II) nitrate hexahydrate (≥ 98.0%, Merck, Germany), cadmium(II) nitrate tetrahydrate (≥ 98.0%, Sigma-Aldrich, Germany), and iron(III) chloride hexahydrate (≥ 98.0%, BDH Chemicals, UK) were obtained from commercial suppliers. Absolute ethanol (99.8%, Sigma-Aldrich, Germany), methanol (99.9%, Merck, Germany), dimethyl sulfoxide-d6 (DMSO-d6, 99.9 atom % D, Cambridge Isotope Laboratories, USA), and deionized water were used for synthesis and analytical procedures.

Instrumentation and analytical equipment

The following instruments were used during the study: a Shimadzu UV-1800 UV-visible spectrophotometer (Shimadzu Corporation, Japan) with a wavelength range of 200–800 nm; a PerkinElmer Spectrum Two FT-IR spectrophotometer (PerkinElmer Inc., USA) operating over the range 4000–400 cm⁻¹; a Bruker Avance III 400 MHz ¹H NMR spectrometer (Bruker BioSpin, Germany); a Stuart SMP30 digital melting point apparatus (Cole-Parmer, UK); a Mettler Toledo ME204E analytical balance (Mettler Toledo, Switzerland) with readability of ± 0.0001 g; a Hanna HI2211 bench pH meter (Hanna Instruments, USA); a thermostatic hot plate equipped with magnetic stirrer; and Class A volumetric glassware for solution preparation and measurements. All instruments were calibrated or verified according to manufacturer recommendations prior to use (Skoog et al., 2018).

Synthesis of the Schiff-base ligand

Salicylaldehyde (0.01 mol, 1.22 g, 1.04 mL) was dissolved in 25 mL absolute ethanol in a 100 mL round-bottom flask. Ethylenediamine (0.01 mol, 0.60 g, 0.67 mL) was separately dissolved in 25 mL ethanol and added dropwise to the salicylaldehyde solution under constant magnetic stirring.

The reaction mixture was refluxed at 70–75 °C for 2 h. After reflux, the solution was allowed to cool to room temperature and then left standing for 30 min to complete crystallization. The yellow precipitate formed was collected by vacuum filtration, washed three times with 5 mL portions of cold ethanol, and dried in a desiccator over silica gel.

The dried product was weighed to determine percentage yield and stored in airtight amber sample bottles until use. The synthesized ligand was obtained as a yellow crystalline solid with 78.4% yield and melting point of 189–191 °C (equation of reaction is shown in Scheme 1).

Scheme 1.

Synthesis of Schiff-base ligand (N,N′-bis(salicylidene)ethylenediamine)

Preparation of lead(II) and metal-ion solutions

A primary Pb²⁺ stock solution (1000 mg/L) was prepared by dissolving an accurately weighed quantity of lead(II) nitrate in deionized water and diluting to 1000 mL in a Class A volumetric flask.

Working Pb²⁺ standard solutions of 0, 5, 10, 20, 30, 40, and 50 mg/L were prepared daily by serial dilution of the stock solution.

Stock solutions of Cu²⁺, Zn²⁺, Ni²⁺, Cd²⁺, and Fe³⁺ (1000 mg/L each) were similarly prepared for selectivity experiments.

For the preparation of the ligand sensing solution, a stock ligand solution (1.0 × 10⁻³ mol/L) was prepared by dissolving an accurately weighed amount of the synthesized ligand in ethanol. The working sensing solution was freshly prepared by dilution as required and protected from prolonged light exposure.

Spectroscopic characterization

FTIR spectroscopy was employed to confirm the formation of the Schiff-base ligand and to investigate coordination behavior upon interaction with Pb²⁺ ions. The characteristic azomethine C=N stretching vibration and phenolic O–H bands were monitored, and shifts in these bands upon metal binding were used as indicators of coordination through nitrogen and oxygen donor atoms (Lever, 1984).

UV-visible spectroscopy was used to investigate the electronic properties of the free ligand and its interaction with Pb²⁺ ions. Absorption bands corresponding to π → π* and n → π* transitions were identified, and spectral changes upon Pb²⁺ addition were monitored. The appearance of ligand-to-metal charge transfer (LMCT) bands was used as evidence of complex formation, consistent with previously reported Schiff-base-based colorimetric sensors (Anand et al., 2022).

¹H NMR spectra were recorded on a Bruker Avance III 400 MHz spectrometer using DMSO-d6 as solvent. Tetramethylsilane (TMS) was used as an internal reference standard. Chemical shifts were reported in parts per million (ppm), and residual solvent peak of DMSO-d6 at δ 2.50 ppm was used for secondary referencing.

Calibration and quantitative analysis

Quantitative evaluation of Pb²⁺ detection was carried out by monitoring absorbance changes at 445 nm, the wavelength corresponding to the LMCT band observed upon Pb²⁺ binding. Calibration curves were constructed by plotting absorbance against Pb²⁺ concentration over the investigated range. Linearity was assessed using regression analysis, and the correlation coefficient (R²) was used to evaluate adherence to the Beer–Lambert law.

Limit of detection (LOD):

$$\mathrm{LOD}=\left(3.3,\sigma \right)/\mathrm{S}$$

Limit of quantification (LOQ):

$$\mathrm{LOQ}=\left(10,\sigma \right)/\mathrm{S}$$

where:

• σ = standard deviation of blank measurements

• S = slope of calibration curve

• Sensitivity was estimated from the slope of the regression line

All measurements were performed in triplicate, and mean absorbance values were used for calibration to improve analytical reliability. The use of Beer–Lambert principles for quantitative spectrophotometric analysis is well established in analytical chemistry and provides a robust basis for concentration determination (Miller & Miller, 2018).

Selectivity studies

Selectivity of the Schiff-base ligand toward Pb²⁺ ions was evaluated in the presence of potentially interfering metal ions commonly found in groundwater. Equal concentrations of Cu²⁺, Zn²⁺, Fe³⁺, Ni²⁺, and Cd²⁺ were added separately to ligand solutions under identical experimental conditions. Absorbance responses and visible color changes were recorded and compared with those observed for Pb²⁺.

The selectivity assessment aimed to establish preferential binding of the ligand toward Pb²⁺ relative to other metal ions, an essential criterion for practical sensing applications. Comparative selectivity studies are standard practice in sensor development and provide insight into coordination preferences and interference tolerance (Callan et al., 2005).

Borehole water sample collection and preparation

Twelve borehole water samples were collected from different locations across Kaduna metropolis to ensure spatial representation. Samples were collected in clean polyethylene bottles that had been pre-rinsed with distilled water and then with the sample water to minimize contamination. The samples were transported to the laboratory and filtered to remove suspended particulates.

Recovery studies were conducted by spiking known concentrations of Pb²⁺ into the borehole water samples. The spiked samples were analyzed using the developed colorimetric method, and percentage recovery and relative standard deviation (RSD) values were calculated to assess method accuracy and precision. This approach is widely employed for validation of analytical methods applied to real environmental samples (Skoog et al., 2018).

Quality assurance and data analysis

All experiments were performed in triplicate, and results are reported as mean ± standard deviation. Analytical precision was evaluated using relative standard deviation values, while accuracy was assessed through recovery studies. Graphical and statistical analyses were performed using standard data analysis tools. The methodological framework adopted in this study conforms to accepted practices for environmental analytical chemistry and method validation (Miller & Miller, 2018).

Methodological limitation

Direct comparative validation against reference instrumental methods such as AAS or ICP-OES was not available during the study period. Such comparative evaluation is recommended in future studies to further establish analytical robustness and trace-level applicability.

Results and discussion

Physicochemical properties of the synthesized Schiff-base ligand

The Schiff-base ligand synthesized via condensation of salicylaldehyde and ethylenediamine was obtained as a yellow crystalline solid with a melting point of 189–191 °C and a percentage yield of 78.4%. The sharp melting range indicates good purity and minimal presence of unreacted starting materials or side products. In Schiff-base chemistry, melting point determination is commonly used as a preliminary purity check, particularly where chromatographic purification is not employed (Shriver & Atkins, 2010). The observed yield is consistent with reported yields for salicylaldehyde-based azomethine ligands synthesized under similar reflux conditions (Callan et al., 2005).

The yellow coloration of the ligand is indicative of extended conjugation involving the azomethine (–C=N–) moiety and the aromatic ring system. Such coloration is typical of Schiff-base compounds and arises from π-electron delocalization, which also underpins their suitability for optical sensing applications (Anand et al., 2022). These physicochemical properties confirm successful ligand formation and provide a suitable basis for subsequent spectroscopic and sensing investigations as shown in Table 1.

Table 1.

Physical and physicochemical properties of the synthesized Schiff-base ligand

Parameter	Observation/value
Physical appearance	Yellow crystalline solid
Odor	Odorless
Melting point (°C)	189–191
Percentage yield (%)	78.4
Solubility	Soluble in ethanol and methanol; sparingly soluble in water
Stability	Stable at room temperature for > 3 months

FTIR spectral characterization of the Schiff-base ligand and Pb²⁺ complex

The FTIR spectrum of the free Schiff-base ligand (Fig. 1 and Table 2) provides clear evidence of successful azomethine formation. The disappearance of the aldehydic C=O stretching vibration of salicylaldehyde (typically observed near 1700 cm⁻¹) and the appearance of a strong band at 1624 cm⁻¹ are characteristic of the azomethine C=N stretching vibration. This observation confirms the condensation reaction between the aldehyde and primary amine groups and is consistent with reported FTIR features of Schiff-base ligands derived from salicylaldehyde (Callan et al., 2005; Lever, 1984).

Fig. 1.

FT-IR spectrum of synthesized Schiff-base ligand

Table 2.

FTIR spectral assignments of the Schiff-base ligand and Pb²⁺ complex

Functional group/vibration	Ligand (cm−1)	Pb2+ complex (cm−1)	Assignment
C=N (azomethine)	1624	1598	Coordination through azomethine N
O–H (phenolic)	3428	3412	Hydrogen bonding/metal interaction
C–O (phenolic)	1258	1234	Coordination via oxygen
Pb–N	—	530	Metal–ligand vibration
Pb–O	—	485	Metal–ligand vibration

Additional bands observed in the FTIR spectrum include broad absorptions in the region of 3200–3400 cm⁻¹ attributable to phenolic O–H stretching vibrations and aromatic C–H stretching bands near 3050 cm⁻¹. The presence of these bands further supports the structural integrity of the ligand and indicates the availability of donor sites for metal coordination.

Upon interaction with Pb²⁺ ions, significant changes were observed in the FTIR spectrum of the ligand (Fig. 2). The azomethine C=N stretching band shifted from 1624 to 1598 cm⁻¹, indicating coordination of the azomethine nitrogen atom to the Pb²⁺ ion. Such downward shifts in C=N stretching frequencies are well documented in Schiff-base metal complexes and are attributed to a reduction in bond order upon coordination (Lever, 1984). In addition, the phenolic O–H band intensity decreased, suggesting involvement of the phenolic oxygen in metal binding, likely through deprotonation and coordination.

Fig. 2.

FT-IR spectrum of synthesized Schiff-base ligand–Pb²⁺ complex

New bands appearing in the low-frequency region around 530 cm⁻¹ and 485 cm⁻¹ were assigned to Pb–N and Pb–O stretching vibrations, respectively. The emergence of these bands provides further confirmation of metal–ligand coordination and supports a bidentate binding mode involving both nitrogen and oxygen donor atoms. Similar coordination behavior has been reported for Pb²⁺ complexes of salicylaldehyde-based Schiff bases in previous studies (Anand et al., 2022).

Overall, the FTIR results confirm successful ligand synthesis and demonstrate effective coordination of Pb²⁺ ions through azomethine nitrogen and phenolic oxygen atoms, as shown in Fig. 2. These findings establish the structural basis for the observed colorimetric response and sensing behavior.

UV-visible spectral properties and electronic transitions

The UV-visible absorption spectrum of the free Schiff-base ligand (Fig. 3) exhibits two prominent absorption bands at 243 nm and 328 nm. The band observed at 243 nm is attributed to π → π* transitions within the aromatic ring system, while the band at 328 nm corresponds to n → π* transitions associated with the lone pair electrons on the azomethine nitrogen atom. These assignments are consistent with established interpretations of electronic transitions in azomethine-containing ligands (Lever, 1984).

Fig. 3.

UV-visible spectrum of synthesized Schiff-base ligand

Upon addition of Pb²⁺ ions to the ligand solution, pronounced changes were observed in the UV-visible spectrum (Fig. 4 and Table 3). The n → π* band exhibited a bathochromic shift, and a new absorption band emerged at 445 nm. This new band is assigned to ligand-to-metal charge transfer (LMCT) transitions, arising from electron donation from ligand donor atoms to the vacant orbitals of the Pb²⁺ ion. The appearance of the LMCT band was accompanied by a visible color change from pale-yellow to orange, which forms the basis of the colorimetric sensing mechanism.

Fig. 4.

UV-visible spectrum of synthesized Schiff base–Pb²⁺ complex

Table 3.

UV-visible absorption bands and electronic transitions

Species	λmax (nm)	Electronic Transition	Assignment
Free ligand	243	π → π*	Aromatic conjugation
Free ligand	328	n → π*	Azomethine nitrogen
Pb2+ complex	368	n → π* (shifted)	Coordination-induced shift
Pb2+ complex	445	LMCT	Ligand-to-metal charge transfer
Visible color	—	—	Yellow → orange

The emergence of LMCT bands in the visible region is a defining feature of Schiff-base-based colorimetric sensors and has been widely exploited for metal-ion detection (Anand et al., 2022). The distinct spectral changes observed upon Pb²⁺ binding indicate strong electronic interaction between the ligand and the metal ion and confirm the suitability of the ligand for optical sensing applications.

¹H NMR confirmation of ligand structure

Additional structural confirmation was obtained from the ¹H NMR spectrum shown in Table 4. A singlet at δ 8.31 ppm was assigned to the azomethine proton (HC=N), confirming formation of the imine linkage. Signals between δ 6.82–7.45 ppm corresponded to aromatic protons of the substituted phenyl ring, while the resonance at δ 3.76 ppm was attributed to methylene protons of the ethylenediamine fragment.

Table 4.

NMR spectral assignments of the synthesized Schiff-base ligand

Chemical shift (δ, ppm)	Multiplicity	Assignment
12.8–13.2	S	Phenolic OH
8.2–8.5	S	Azomethine proton (HC=N)
6.8–7.5	M	Aromatic protons
3.6–3.9	s/t	–CH2–CH2– linker protons
2.50	S	Residual DMSO-d6 solvent peak

A strongly downfield signal at δ 12.94 ppm was assigned to the phenolic hydroxyl proton, likely stabilized by intramolecular hydrogen bonding. Similar proton environments have been reported for salicylaldehyde-derived Schiff-base ligands characterized by modern NMR techniques.

The proton nuclear magnetic resonance (¹H NMR) spectrum of the synthesized Schiff-base ligand (derived from salicylaldehyde and ethylenediamine) recorded in DMSO-d₆ is characterized by diagnostic signals corresponding to the phenolic hydroxyl proton, azomethine proton, aromatic ring protons, and methylene linker protons.

Table 4 represents the ¹H NMR spectral assignments of the synthesized Schiff-base ligand in DMSO-d₆. The presence of the azomethine proton near δ 8.3 ppm and the phenolic OH resonance near δ 13 ppm provide strong evidence of successful Schiff-base formation.

Calibration curve and quantitative performance

Quantitative evaluation of Pb²⁺ detection was carried out by monitoring absorbance changes at 445 nm, corresponding to the LMCT band (Fig. 5). The calibration curve constructed by plotting absorbance against Pb²⁺ concentration exhibited excellent linearity over the investigated concentration range, with a correlation coefficient (R²) of 0.995. This high degree of linearity confirms adherence to the Beer–Lambert law and indicates reliable quantitative performance (Miller & Miller, 2018).

Fig. 5.

Calibration curve of Pb²⁺ detection

The linear response demonstrates that the colorimetric sensor can be effectively used for quantitative estimation of Pb²⁺ within the tested range. Although more sophisticated techniques such as ICP-MS offer lower detection limits, the performance observed here is adequate for preliminary screening and monitoring applications, particularly in settings where access to advanced instrumentation is limited. Similar linear calibration behavior has been reported for other Schiff-base-based Pb²⁺ sensors, reinforcing the validity of the approach (Anand et al., 2022).

Benesi–Hildebrand analysis and binding stoichiometry

To further elucidate the binding interaction between the Schiff-base ligand and Pb²⁺ ions, Benesi–Hildebrand analysis was employed (Fig. 6). The linearity of the Benesi–Hildebrand plot indicates a 1:1 binding stoichiometry between the ligand and Pb²⁺ ions. This stoichiometric relationship is consistent with the bidentate coordination mode inferred from FTIR analysis and is commonly observed in Schiff base–metal complexes (Lever, 1984).

Fig. 6.

Benesi–Hildebrand plot for Pb²⁺ binding

The confirmation of 1:1 binding provides insight into the sensing mechanism and supports the use of simple equilibrium models for quantitative interpretation. The application of Benesi–Hildebrand analysis is well established in spectrophotometric studies of metal–ligand interactions and lends additional credibility to the analytical framework adopted in this study (Miller & Miller, 2018).

Selectivity toward Pb²⁺ in the presence of competing metal ions

Selectivity is a critical parameter for practical sensing applications, particularly in complex environmental matrices where multiple metal ions may coexist. The selectivity of the Schiff-base ligand toward Pb²⁺ was evaluated in the presence of Cu²⁺, Zn²⁺, Fe³⁺, Ni²⁺, and Cd²⁺ ions under identical experimental conditions (Table 5).

Table 5.

Selectivity study of the Schiff-base ligand

Metal ion	λmax (nm)	Absorbance	Visible change	Inference
Pb2+	445	0.865	Yellow → orange	Strong response
Cu2+	410	0.238	Faint yellow	Weak interference
Zn2+	386	0.210	None	No response
Fe3+	396	0.192	Pale-brown	Minimal response
Ni2+	402	0.205	None	No response
Cd2+	388	0.176	None	No response

The results demonstrate a markedly higher absorbance response and more pronounced color change for Pb²⁺ compared to other tested metal ions. While minor spectral changes were observed in the presence of some competing ions, these responses were significantly lower than those induced by Pb²⁺. This preferential response suggests favorable coordination geometry and electronic compatibility between the ligand and Pb²⁺ ions.

The observed selectivity can be rationalized based on the soft acid–base interactions between Pb²⁺ and the donor atoms present in the ligand. Pb²⁺, being a borderline soft metal ion, exhibits strong affinity for nitrogen and oxygen donors, particularly in flexible ligand environments that can accommodate its relatively large ionic radius. Similar selectivity trends have been reported in previous studies of Schiff-base-based Pb²⁺ sensors (Anand et al., 2022).

Application to borehole water samples from Kaduna metropolis

The practical applicability of the developed colorimetric sensor was evaluated using twelve borehole water samples collected from different locations across Kaduna metropolis (Table 6). Recovery studies were performed by spiking known concentrations of Pb²⁺ into the samples, followed by analysis using the developed method.

Table 6.

Determination and recovery of Pb²⁺ in borehole water samples (n = 12)

Sample code	Location	Pb2+ found (mg L−1 ± SD)	Pb2+ spiked (mg L−1)	Recovery (%)	RSD (%)
BH1	Barnawa	0.47 ± 0.011	0.50	94.0	2.3
BH2	Kakuri	0.49 ± 0.008	0.50	98.0	1.6
BH3	Kawo	0.46 ± 0.012	0.50	92.0	2.6
BH4	Tudun Wada	0.48 ± 0.009	0.50	96.0	1.9
BH5	Ungwan Rimi	0.48 ± 0.010	0.50	96.0	2.1
BH6	Malali	0.47 ± 0.009	0.50	94.0	1.9
BH7	Narayi	0.49 ± 0.007	0.50	98.0	1.4
BH8	Rigasa	0.46 ± 0.011	0.50	92.0	2.4
BH9	Sabon Tasha	0.48 ± 0.010	0.50	96.0	2.1
BH10	Kabala	0.47 ± 0.009	0.50	94.0	1.9
BH11	Gonin Gora	0.49 ± 0.008	0.50	98.0	1.6
BH12	Mando	0.48 ± 0.010	0.50	96.0	2.1

The recovery values obtained ranged from 92 to 98%, with relative standard deviation (RSD) values below 3%. These results indicate good analytical accuracy and precision and demonstrate that the sensor performs reliably in real water matrices. The low RSD values reflect good repeatability and minimal matrix interference, which is essential for practical environmental monitoring.

Comparable recovery ranges have been reported for colorimetric Pb²⁺ sensors applied to environmental water samples, further supporting the robustness of the method (Anand et al., 2022). Although centralized instrumental methods such as AAS and ICP-MS remain indispensable for confirmatory analysis, the present method offers a valuable complementary approach for rapid screening and preliminary assessment.

Comparison with existing Nigerian and global studies

The findings of this study align well with existing literature on lead detection using Schiff-base ligands and colorimetric sensors. Nigerian groundwater studies have consistently highlighted the presence and potential risks of lead contamination, emphasizing the need for accessible monitoring tools (Akan et al., 2014; Sadiq et al., 2020). However, most of these studies rely on centralized laboratory techniques, which limit monitoring frequency and spatial coverage.

Globally, Schiff-base-based colorimetric sensors have been widely investigated for metal ion detection, with reported advantages including simplicity, low cost, and visual detection capability (Anand et al., 2022; Callan et al., 2005). The present study contributes to this body of work by demonstrating successful application in a Nigerian urban context, thereby addressing a key gap in localized environmental sensing research.

The WHO guideline value for lead in drinking water is 0.01 mg/L. While the present method is designed primarily for rapid preliminary screening rather than ultra-trace quantification, it can serve as an early-warning tool for identifying borehole sources requiring confirmatory laboratory analysis. A limitation of this study is the absence of direct validation using AAS or ICP-OES, which is recommended for future work.

Comparison with existing methods

To evaluate the practical relevance of the developed Schiff-base colorimetric sensor, its analytical and operational performance was compared with selected conventional and emerging methods for Pb²⁺ detection, as presented in Table 7. The comparison considered sensitivity, approximate cost, portability, and suitability for routine environmental monitoring.

Table 7.

Comparison with existing methods

Method	Sensitivity	Cost	Portability	Suitability
ICP-MS	Excellent	Very high	No	Laboratory
AAS	High	High	No	Laboratory
Electrochemical Sensors	High	Moderate	Moderate	Field/lab
Fluorescent sensors	High	Moderate	Low	Laboratory
Present Schiff-base sensor	Moderate	Low	Excellent	Rapid screening

The results indicate that instrumental techniques such as ICP-MS and AAS remain superior in ultra-trace quantification and regulatory confirmation analysis. However, they are associated with high capital cost, specialized operation, and limited field portability. In contrast, the present Schiff-base sensor offers clear advantages in affordability, ease of operation, and rapid on-site preliminary screening, making it particularly useful for groundwater monitoring in resource-limited settings.

Implications for groundwater monitoring in resource-limited settings

The results of this study have important implications for groundwater monitoring in resource-limited environments. The developed colorimetric sensor provides a simple and cost-effective means of preliminary Pb²⁺ screening that can complement conventional analytical methods. Its application to borehole water samples from Kaduna metropolis demonstrates its potential utility for community-level monitoring and early detection of contamination.

While the method does not replace advanced instrumental techniques, it offers a practical screening tool that can support more frequent monitoring and inform decision-making regarding water quality management. Such approaches are increasingly recognized as valuable components of integrated environmental monitoring strategies, particularly in developing regions (WHO, 2023).

Implications, limitations, and conclusions

Environmental and public health implications

The findings of this study have important implications for environmental monitoring and public health protection, particularly in urban and peri-urban regions of developing countries where reliance on groundwater is high. Borehole water constitutes a major source of domestic water supply for millions of people in Nigeria and across sub-Saharan Africa. In such contexts, undetected lead contamination poses a serious risk, given the cumulative and irreversible health effects associated with chronic exposure to Pb²⁺, especially among vulnerable populations such as children and pregnant women (Flora et al., 2012; WHO, 2023).

The successful application of the developed Schiff-base colorimetric sensor to borehole water samples from Kaduna metropolis demonstrates the potential of simple chemical sensing tools to complement existing groundwater quality assessment frameworks. By enabling rapid, low-cost, and visually interpretable detection of Pb²⁺, the sensor provides an accessible means for preliminary screening of water sources that may otherwise go unmonitored due to logistical or financial constraints. This is particularly relevant in settings where centralized laboratory facilities are limited or where routine sampling and analysis are not feasible.

From a broader environmental management perspective, the use of such screening tools can support early detection of contamination hotspots and inform targeted interventions. Preliminary identification of elevated lead levels in groundwater can prompt further confirmatory analysis using advanced instrumental techniques and facilitate timely public health responses. In this way, colorimetric sensors may play a supportive role within tiered monitoring strategies that integrate field-level screening with laboratory-based confirmation, an approach increasingly advocated in global water quality management frameworks (WHO, 2023).

Relevance to groundwater monitoring in developing countries

The relevance of this work extends beyond Kaduna metropolis to other urban centers in Nigeria and similar settings across Africa and other developing regions. Many cities face comparable challenges, including rapid urbanization, aging infrastructure, informal industrial activities, and limited regulatory oversight of groundwater quality. Studies conducted in different parts of Nigeria and Africa have consistently reported variable physicochemical parameters and trace metal contamination in groundwater, underscoring the need for more frequent and decentralized monitoring approaches (Adelana & MacDonald, 2008; Akan et al., 2014).

The Schiff-base colorimetric sensor developed in this study aligns with the growing emphasis on context-appropriate analytical solutions. Its synthesis relies on readily available reagents, and its operation does not require complex instrumentation, making it suitable for adaptation in academic laboratories, environmental agencies, and potentially community-based monitoring initiatives. While the present study was conducted under controlled laboratory conditions, the demonstrated performance in real borehole water samples suggests that such sensors could be further optimized for semi-field or field-deployable applications.

Limitations of study

Despite the encouraging results obtained, certain limitations of the present study should be acknowledged. First, the colorimetric sensor developed here is intended as a screening tool rather than a replacement for highly sensitive instrumental techniques such as AAS or ICP-MS. While the method exhibits good linearity, selectivity, and recovery within the investigated concentration range, its detection capability may not match the ultra-trace sensitivity achievable with advanced spectrometric methods (Skoog et al., 2018).

Second, the selectivity studies were conducted against a limited number of competing metal ions commonly encountered in groundwater. Although the ligand demonstrated preferential response toward Pb²⁺ under the tested conditions, the presence of complex matrices or higher concentrations of interfering species could potentially influence sensor performance. Further studies involving a broader range of ions and varying water chemistries would provide additional insight into interference effects.

Third, the study was limited to twelve borehole water samples collected within Kaduna metropolis. While this number provides meaningful spatial representation and validates real-sample applicability, larger-scale sampling across different seasons and hydrogeological settings would strengthen the generalizability of the findings. Seasonal variations in groundwater chemistry, influenced by rainfall and anthropogenic activity, may also affect metal speciation and sensor response.

Future perspectives and research directions

Future research could build on the findings of this study in several ways. Structural modification of the Schiff-base ligand through functional group substitution may enhance selectivity or sensitivity toward Pb²⁺ and potentially allow discrimination among different heavy metal ions. Incorporation of the ligand into solid-state platforms, test strips, or polymer matrices could facilitate development of portable sensing devices suitable for on-site analysis.

Additionally, coupling colorimetric sensing with digital image analysis using smartphone-based platforms represents a promising avenue for semi-quantitative field measurements. Such approaches could improve data recording, facilitate community engagement, and support real-time reporting of water quality information. Integration of simple sensors into broader monitoring networks aligns with global efforts to improve access to safe drinking water and strengthen environmental surveillance systems (WHO, 2023).

Conclusions

In this study, a Schiff-base ligand synthesized from salicylaldehyde and ethylenediamine was developed and evaluated as a colorimetric sensor for Pb²⁺ detection in groundwater. Spectroscopic characterization using FTIR and UV-visible techniques confirmed azomethine formation and coordination of Pb²⁺ ions through nitrogen and oxygen donor atoms. The interaction between the ligand and Pb²⁺ produced distinct spectral changes, including the emergence of a ligand-to-metal charge transfer band accompanied by a visible color change that enabled straightforward detection.

The sensor demonstrated reliable quantitative performance within the investigated concentration range, exhibiting linear Beer–Lambert behavior (R² = 0.995), preferential response toward Pb²⁺ in the presence of selected competing metal ions, and satisfactory accuracy and precision when applied to borehole water samples from Kaduna metropolis. Recovery values ranging from 92 to 98% with low relative standard deviations indicate consistent performance of the method under the tested experimental and sample conditions.

Although the method does not replace advanced instrumental techniques for trace-level quantification, it offers a practical, low-cost, and accessible approach for preliminary screening of lead contamination in groundwater. The findings highlight the potential applicability of Schiff-base-based colorimetric sensors as complementary tools for groundwater monitoring in resource-limited environments and contribute to ongoing efforts to develop context-appropriate analytical methods for environmental assessment.

Author contribution

A.A.O.supervised the research, contributed to the study conceptualization and methodology, provided guidance on data interpretation, and critically revised the manuscript for important intellectual content. A.S. conducted the experimental work, sample collection, data analysis, and wrote the original draft of the manuscript. A. A. O  Co Supervisor. Plotted the graphs  and contributed to data interpretation and revision of the recommendations and contributions to knowledge. All authors reviewed and approved the final version of the manuscript.

Funding

This research received no funding.

Data availability

The data is available and will be shared with the public upon reasonable request.

Declarations

Conflict of interest

The authors declare no competing interests.

Clinical trial number

Not applicable.