Fluorescence sensing and adsorption kinetics of Gd-doped AgInS₂ I-III-VI quantum dots – A case study of Ag⁺ ions interactions

Abstract

The poor fluorescence properties of magneto-fluorescent paramagnetic-ion (Gd, Mn, or Co) doped I-III-VI quantum dots (QDs) at higher paramagnetic-ion doping concentrations have limited their use in magnetic-driven water-based applications. This work presents, for the first time, the use of stable magneto-fluorescent Gd-doped AgInS₂ QDs at high Gd mole ratios of 16, 20, and 30 for the fluorescence detection and adsorption of Ag⁺ ions in water environments. The effect of pH, initial concentration, contact time, and adsorbent dosage were systematically evaluated. The AgInS₂ QDs with the least Gd mole ratio (16) exhibited the best fluorescence characteristics (LOD = 0.88, R² = 0.9549) while all materials showed good adsorption properties under optimized conditions (pH of 2, initial concentration of 30 ppm, contact time of 10 min and adsorbent dosage of 0.02 g) and a pseudo 2nd order reaction was followed. The adsorption mechanism was proposed to be a combination of ion-exchange, electrostatic interaction, complexation, and diffusion processes. Application in environmental wastewater samples revealed complete removal of Ag ⁺ ions alongside Ti²⁺ Pb²⁺, Ni²⁺, Cr³⁺, and Zn²⁺ ions.

Graphical abstract

1. Introduction

The increased cases of environmental pollution have led to stricter regulations for water quality assessments in industrial effluents. Water quality assessments commonly involve the concentration evaluations of heavy metals such as Pb, Hg, Cd, As, Cu, Cr, Ni, Fe, and Zn [1,2]. Other heavy metals like Ag, Au, Mo, and Co are less popularly studied but they still require removal since they can be toxic at elevated concentrations. Ag remains one of the least studied heavy metals in wastewater yet it is toxic for many biological systems [3]. Moreover, the emergence of Ag-containing products in industries (i.e. anti-bacterial products, textiles, and electronics) and medicine (i.e. medical devices, anti-cancer treatments) has increased the occurrence of Ag in the environment [3,4]. Therefore, rapid Ag⁺ ion detection and removal approaches at wide concentration ranges are crucial.

The quest for applying safe and sustainable magneto-fluorescent materials for various applications other than the ever-exploited biological use has been a subject of current research interest. As a result, the ternary I-III-VI quantum dots (QDs) are currently being evaluated as fluorescent components of magneto-fluorescent materials for water quality monitoring and remediation studies [5,6]. Besides their safety compared to conventional binary II-VI QDs [[7], [8], [9]], the presence of the paramagnetic ions (Gd, Mn, Ni, or Co) and magnetic materials (Fe₃O₄) within their current structure enables them for potential use for easy separation from aqueous solutions via magnetic field attractions [10]. Ternary I-III-VI quantum dots have demonstrated promising potential in the fluorescence detection of various heavy metals [11,12] but rarely in the detection of Ag [13]. Furthermore, a few challenges such as low magnetic loading capacity due to loss in fluorescence at high magnetic loading and stability at certain pHs are eminent [14,15], limiting their use for water remediation. Over the years, a few authors explored the development of magneto-fluorescent Fe₃O₄@I-III-VI QD nanocomposites for fluorescence detection of heavy metals in water [5,6]. For example, Kurshanov et al. fabricated a magneto-fluorescent microsphere based on CaCO₃–Fe₃O₄–AgInS₂/ZnS for the detection of Co²⁺, Ni²⁺, and Pb²⁺ ions in water. The microsphere was prepared by doping CaCO₃ microspheres with Fe₃O₄ NPs, followed by capping with various layers of polymer and depositing the TGA-capped AgInS₂/ZnS QDs [5]. In another development, Liu et al. fabricated oligonucleotide-capped CuInS₂@Fe₃O₄ nanocomposite by conjugating Fe₃O₄ NPs with oligonucleotide-capped CuInS₂ QDs for fluorescence detection of arsenate in water [6]. Although both nanocomposites displayed magnetic functions which could offer high adsorption capacities and easy separation for the removal of heavy metals, adsorption studies were not reported. Moreover, the preparation of the nanocomposites involved several steps for the synthesis and conjugation of the multiple components. Recently, we reported a one-pot synthesis of stable magneto-fluorescent citrate-capped Gd-doped AgInS₂ QDs at high Gd loading (up to 20 Gd mol ratio) with enhanced photoluminescence characteristics [16]. In this work, citrate-capped Gd-doped AgInS₂ (AIS) QDs were synthesized up to a Gd mole ratio of 30. For the first time, we present paramagnetic ion–doped I-III-VI QDs-Gd-doped AgInS₂ QDs for the fluorescence detection and removal of Ag ⁺ ions in aqueous solutions. To describe the potential for real life application, the adsorption isotherms, kinetics and interferences of other heavy metals present in industrial wastewater samples were discussed.

2. Material and methods

All chemicals (Sigma-Aldrich, South Africa) were used as purchased. Deionized water was used for all aqueous preparations.

2.1. Synthesis of the quantum dots

Gd-doped AIS QDs were prepared following our previous report [16]. Typically, 0.0453 g of Gd₂O₃ (0.1248 mmol) was dissolved in 300 μL HCl under mild heat and made up to 10 mL with deionized water. The Gd₂O₃ solution was added to a AgNO₃/InCl₃/sodium citrate/thioglycolic acid (TGA) mixture (15 mL) in another beaker at pH 6 in the presence of Na₂S followed by refluxing at 95 °C under stirring. The materials were centrifuged (5000 rpm), washed with ethanol, and dried at 80 °C for 1 h.

2.2. Detection and adsorption studies

Ag⁺ ion solutions were diluted using deionized water and the pH was adjusted using dilute hydrochloric acid and sodium hydroxide. 0.02 g of adsorbent material and 30 mL of Ag⁺ ion solutions were added to glass vials and shaken for 10 min at 200 rpm. This was followed by fluorescence analysis at an excitation wavelength of 375 nm at room temperature. The limit of detection (LOD) was calculated according to the literature [17]. The fluorescence intensity of Gd-AIS QDs PL peak was measured 5 times in the presence and absence of Ag⁺ at various concentrations and the standard deviations were achieved. The slope was obtained from the plot of fluorescence intensity vs Ag⁺ concentration. Thus, the LOD was estimated using equation (1).

$$\mathrm{L}\mathrm{O}\mathrm{D}=3.3\left(\sigma \right)/\mathrm{S}$$ (1)

where σ is the standard deviation of the response of the Gd-AIS QDs fluorescence intensity and S is the slope of the plot of Gd-AIS QDs fluorescence intensity in the absence and presence at Ag ⁺ at various concentrations (calibration curve).

The adsorbent was collected with a magnet. The Ag⁺ solutions were analyzed using an Agilent 5900 ICP-OES before and after adsorption. The % removal and amount of heavy metal ions adsorbed (Q_e) were calculated by equations (2), (3) respectively.

$${\mathrm{Q}}_{\mathrm{e}}=\frac{Co-Ce}{m}.\mathrm{V}$$ (2)

$$\%\mathrm{R}\mathrm{e}\mathrm{m}\mathrm{o}\mathrm{v}\mathrm{a}\mathrm{l}=\frac{Co-Ce}{Co}.100$$ (3)

where Q_e is the amount of heavy metal ion adsorbed at equilibrium (mg/g), C_o and C_e are the initial and equilibrium concentrations of heavy metal ions solutions (mg/L), V is the volume of solution (L) and m is the mass of the adsorbent (g).

Acidified water samples (Ag-WS) were collected during a Ag recovery process from an industrial company. The acidified water samples were adjusted to pH 2 using dilute nitric acid.

The adsorption studies were subjected to isotherm models such as Langmuir and Freudlich as well as kinetic models such as pseudo-1st and pseudo-2nd orders to investigate the adsorption mechanism.

2.2.1. Langmuir model: adsorption at homogenous sites

$${\mathrm{C}}_{\mathrm{e}}/{\mathrm{Q}}_{\mathrm{e}}=(1/\mathrm{K})(1/{\mathrm{Q}}_{\mathrm{M}})+(1/{\mathrm{Q}}_{\mathrm{M}})\left(\mathrm{C}\mathrm{e}\right)$$ (4)

Where Ce is the concentration of the metal ion at the equilibrium (mg/L), Q_e is the amount of metal ion adsorbed per mass of the nanocomposite (adsorbent) (mg/g), K is the equilibrium constant and Q_M is the amount of metal ion required to form a monolayer.

The plot of Ce/Q_e versus Ce typically gives a straight line whose slope = (1/Q_M) and intercept = (1/K) (1/Q_M).

2.2.2. Freundlich model: adsorption at heterogeneous sites

$$\mathrm{Log}{\mathrm{Q}}_{\mathrm{e}}=\log \mathrm{K}+1/\mathrm{n}\log {\mathrm{C}}_{\mathrm{e}}$$ (5)

Where n is an empirical constant. A plot of log Q_e versus log C_e gives a straight line with a slope = 1/n and an intercept = log K.

2.2.3. The pseudo-first-order kinetics

$$\ln ({\mathrm{Q}}_{\mathrm{e}}–{\mathrm{Q}}_{\mathrm{t}})=\ln \left({\mathrm{Q}}_{\mathrm{e}}\right)–\mathrm{K}.\mathrm{t}$$ (6)

Where Q_t is the amount of heavy metals adsorbed at a time (t).

2.2.4. The pseudo-second-order kinetics

$$\mathrm{t}/{\mathrm{Q}}_{\mathrm{t}}=1/\mathrm{h}+\mathrm{t}/{\mathrm{Q}}_{\mathrm{e}}\mathrm{W}\mathrm{h}\mathrm{e}\mathrm{r}\mathrm{e}\mathrm{h}=\mathrm{K}{\mathrm{Q}}_{\mathrm{e}}^2$$ (7)

2.3. Characterization

The spectrofluorophotometer (RF-6000, Shimadzu, Japan) was utilized for the fluorescence analysis. Fourier transform infrared spectroscopy (FTIR) spectra were measured using a PerkinElmer Spectrum Two UATR-FTIR spectrometer. The morphology of the samples was analyzed using a JEOL electron microscope (JEM-2100) at an accelerating voltage of 200 kV. Surface zeta potential measurements were carried out using a Malvern Zetasizer Nano at 25 °C. An Agilent 5900 inductively coupled plasma optical emission spectroscopy (ICP-OES) and Spectro Xepos05 energy-dispersive X-ray fluorescence (ED-XRF) instruments were used to determine the Gd, Ag, In, and S concentrations of the as-synthesized materials.

3. Results and discussion

3.1. Synthesis of Gd-AIS QDs and fluorescence detection

The Gd-doped AgInS₂ QDs were synthesized with a 7:16 TGA: sodium citrate mole ratio suggesting that the surface functionality is dominated by carboxylate groups from citrate molecules and a few from the TGA molecules hence the materials are referred to as “citrate-capped Gd-doped AgInS₂ QDs”. At such high Gd loading the carboxylate groups on the AIS QD surface coordinate with Gd³⁺ ions and passivates the surfaces, filling the defect sites and thus the stable fluorescence properties [16]. The elemental analysis (Table 1) of the Gd-AIS QDs showed an obvious increase in the Gd concentration at increased Gd mole ratios while the slight decreases in Ag and In were attributed to possible cation-exchange occurring during the Gd doping when Gd diffuses into the AgInS₂ QDs lattice.

Table 1.

Elemental analysis of the Gd-AIS QDs as increased Gd mole ratios.

Elements	Gd (%)	Ag (%)	In (%)	S (%)
Gd16-AIS	29.16	0.392	1.24	2.02
Gd20-AIS	39.91	0.353	0.979	2.22
Gd30-AIS	45.87	0.359	0.938	2.00

The results of the fluorescence evaluation of all Gd-AIS QDs (Gd mole ratios 16, 20, and 30) after Ag⁺ ions interactions revealed a slight PL enhancement at low concentrations and gradual PL quenching at higher concentrations (Fig. 1A–G). These behaviors were due to the initial passivation of the surface defects at low concentrations and saturation of surface traps at high concentrations, which destabilized the QDs hence the quenching [18,19]. Moreover, the excess Ag⁺ interacts with the citrate functionality on the QD surface, which reduces the Ag ⁺ to Ag⁰ (nanoparticles), further destabilizing the QDs.

Fig. 1.

PL spectra and comparative Stern-Volmer's plot of Gd-AIS QDs at Gd mole ratios of 16 (A–C), 20 (D–E), and 30 (F–G) after adsorption of Ag⁺ ions at increased concentrations (1, 3, 5, 7, 10, 30, 50, 70, 100 ppm). Excitation wavelength: 375 nm.

Also, the exhibited slight blue-shift was associated with Ag⁺ ion diffusion into the QD lattice as a result of cation-exchange (Fig. 1A–G and Table 2). However, among all materials, Gd-doped AIS QDs with a Gd mole ratio of 16 showed the best limit of detection (LOD) with a correlation coefficient (R²) of 0.9549 (Table 3). The lower LOD obtained with Gd16-AIS QDs compared to Gd20-AIS QDs and Gd30-AIS QDs could be due to their larger number of surface defects due to the lower degree of Gd-citrate passivation present on the QD surface [20] thus appreciating even the smallest concentration of Ag⁺ ion to passivate existing defects hence the PL enhancement at Ag+ion low concentrations.

Table 2.

Photoluminescence properties of Gd-AIS QDs at varied Gd mole ratios after interaction with Ag⁺ ions at increased concentrations.

Gd mole ratio	16		20		30
Sample name	PL intensity (a. u.)	PL peak (nm)	PL intensity (a. u.)	PL peak (nm)	PL intensity (a. u.)	PL peak (nm)
Gd16-AIS QDs	129.3	603	89.52	579	103.6	575
Gd-AIS QDs + 1 ppm	431.8	587	122.1	555	280.1	579
Gd-AIS QDs + 3 ppm	494.8	587	138.2	543	261	579
Gd-AIS QDs + 5 ppm	557.8	587	149	543	239	583
Gd-AIS QDs + 7 ppm	620.8	587	161.7	543	218	571
Gd-AIS QDs + 10 ppm	683.8	587	150	543	142.2	559
Gd-AIS QDs + 30 ppm	765.1	587	186.6	543	–	–
Gd-AIS QDs + 50 ppm	–	–	–	–	–	–
Gd-AIS QDs + 70 ppm	–	–	–	–	–	–
Gd-AIS QDs + 100 ppm	–	–	–	–	–	–

Table 3.

Comparative Ag ⁺ ion detection characteristics of Gd-AIS QDs at Gd mole ratios 16, 20, and 30.

Material	LOD (ppm)	R2	Range (ppm)	λ max (nm)
Gd16-AIS QDs	0.88	0.9549	1–10	575
Gd20-AIS QDs	2.36	0.7007	1–30	547
Gd30-AIS QDs	1.23	0.8351	1–30	547

3.2. Adsorption properties – effects of pH, concentration, contact time, and adsorbent dosage

Generally, lower pH values (2 and 4) produced higher % Ag removals than higher pH (6) for all adsorbents, except for Gd30-AIS QDs which showed a decrease in % removal at pH 2 compared to pH 4 and 6. The superiority of the lower pH on the removal of Ag⁺ ions over the higher ones can be explained based on the tendency of the carboxylate ions of the citric acid to be protonated at low pHs (pKa1 = 3.13, pKa2 = 4.76, and pKa3 = 6.40), causing loss of some of the carboxylate groups, thus aggregation of the QDs and some part of the surface of the QDs available for the adsorption of Ag⁺ ions. Comparatively, all Gd-AIS QDs produced almost 100% removal throughout the studied pH range (Fig. 2A). Ag removals above 95% from 1 to 100 ppm were exhibited by Gd16-AIS and Gd20-AIS QDs while Gd30-AIS QDs showed a slight decrease to 90% at 100 ppm (Fig. 2B). The slight decrease might be due to the lower number of carboxylic acid groups available on the Gd-AIS QD surface at a higher Gd mole ratio compared to QDs with a lower Gd mole ratio.

Fig. 2.

Adsorption of Ag ⁺ using Gd-AIS (Gd mole ratio 16, 20, and 30) - Effects of pH (2, 4, and 6) (adsorbent dosage = 0.02 g and initial Ag⁺ ion concentration = 30 ppm, contact time = 10 min) (A), increased initial Ag⁺ ion concentration (1–100 ppm) (dosage = 0.02 g, contact time = 10 min and pH) (B), contact time (1, 2, 3, 4, 5, 10, 30, 45 and 60 min) (pH = 2, adsorbent dosage = 0.02 g and initial Ag⁺ ion concentration = 30 ppm) (C) and increased adsorbent dosage (0.005, 0.01, 0.02, 0.03 and 0.04 g) (pH = 2, contact time = 10 min and initial Ag⁺ ion concentration = 30 ppm) (D).

The contact time studies revealed that maximum adsorption of Ag⁺ ions could be achieved within 1 min for all Gd-AIS QDs (Fig. 2C). % Removal values of 98%, 96% and 94% were exhibited by Gd16-AIS, Gd20-AIS and Gd30-AIS QDs respectively. The rapid uptake of Ag⁺ ions was attributed to the sufficient number of binding sites on the surface of the QDs.

The study on the effect of adsorbent dosage for Ag⁺ ions adsorption showed that adsorption of Ag⁺ ions increased with increased adsorbent dosage from 0.005 g to 0.02 g and was kept constant at higher dosages for all Gd-AIS QDs (Fig. 2D). However, Gd30-AIS QDs showed a significantly lower % removal (63%) at the lowest dosage (0.005 g) compared to Gd16-AIS QDs and Gd20-AIS QDs (92% and 95%). This was also associated with the lower number of carboxylic acid groups available for binding on the QDs at a higher Gd mole ratio.

3.3. Adsorption isotherm and kinetics studies

Langmuir and Freundlich isotherm models (equations (4), (5))) were applied to study the adsorption characteristics for Ag⁺ ions removal using the Gd-AIS QDs at increased Gd mole ratios (16, 20, and 30). As seen in Fig. 2B, the adsorbents showed % Ag removal above 95% for the studied Ag⁺ ion concentration range. Thus, it was not possible to obtain a linear plot of Ce/Qe vs Ce (Langmuir) and log Qe vs log Ce (Freundlich) with good R² values since the Ce values were constant throughout the concentration range (Fig. 3A–F, Table 4, Table 5). Moreover, the maximum adsorption capacities (Q_M) obtained experimentally decreased from 1503 mg/g (for Gd16-AIS and Gd20-AIS QDs) to 1392 mg/g (Gd30-AIS QDs) (Table 4), which is much higher than most reported adsorbent materials. The Gd16-AIS QDs not only offer high adsorption capabilities but also offer simultaneous fluorescent detection of Ag⁺ ions at wider concentration ranges and comparable LOD (Table 3) compared to other materials (Table 6).

Fig. 3.

Langmuir and Freundlich isotherms for Ag ⁺ ions adsorption onto Gd16-AIS QDs (A–B), Gd20-AIS QDs (C–D), and Gd30-AIS QDs (E–F).

Table 4.

Langmuir and Freundlich isotherm parameters for adsorption of Ag ⁺ ions onto Gd16-AIS QDs, Gd20-AIS QDs, and Gd30-AIS QDs.

% Ag removal	Co (ppm)	Ce (ppm)	Qe (mg/g)	Ce/Qe	Log Ce	Log Qe
Gd16-AIS QDs
99.89	0.89	0.05	12.6	0.003968	−1.301029996	1.100370545
99.89	9.65	0.05	144	0.000347	−1.301029996	2.158362492
99.89	29.55	0.05	442.5	0.000113	−1.301029996	2.645913275
99.89	47.61	0.05	713.4	0.000070	−1.301029996	2.853333105
99.89	68.82	0.05	1031.55	0.000048	−1.301029996	3.013490283
99.89	100.26	0.05	1503.15	0.000033	−1.301029996	3.177002321
Gd20-AIS QDs
99.89	0.89	0.05	12.6	0.003968	−1.301029996	1.100370545
96.99	9.65	0.29	140.4	0.002066	−0.537602002	2.147367108
99.63	29.55	0.055	442.425	0.000124	−1.259637311	2.64583966
99.89	47.61	0.05	713.4	0.000070	−1.301029996	2.853333105
99.89	68.82	0.05	1031.55	0.000048	−1.301029996	3.013490283
99.89	100.26	0.05	1503.15	0.000033	−1.301029996	3.177002321
Gd30-AIS QDs
99.89	0.89	0.05	12.6	0.003968254	−1.301029996	1.100370545
97.92	9.65	0.25	141	0.00177305	−0.602059991	2.149219113
99.89	29.55	0.05	442.5	0.000112994	−1.301029996	2.645913275
99.89	47.61	0.05	713.4	7.00869E-05	−1.301029996	2.853333105
99.03	68.82	0.665	1022.325	0.000650478	−0.177178355	3.009588981
92.58	100.26	7.44	1392.3	0.005343676	0.871572936	3.143732823

Table 5.

Freundlich and Langmuir parameters for Ag ⁺ ions adsorption isotherms.

Langmuir				Freundlich
Adsorbent	Qm (mg/g)	KL	R2	1/n	KF	R2
Gd16-AIS QDs	65.36	1.7 × 1016	0.0387	1.915	1	0.6438
Gd20-AIS QDs	169.49	14.75	0.1309	1.670	2.909	0.5547
Gd30-AIS QDs	1667	0.6	0.553	0.106	153.6	0.0057

Table 6.

Comparative LODs and maximum adsorption capacities for Ag ⁺ ions using various materials.

Material	Detection/removal	Range (mg/L)	LOD (mg/L)	Qm (mg/g)/%removal	References
Gd16-AgInS2 QDs	Yes/Yes	1–100	0.88	1503 mg/g	This work
AgInSZn QDs	Yes/No	30–100	28.01	–	[13]
Fluorescent carbon nanosheets	Yes/Yes	0–20 30–120	1.74 16.16	98%	[13]
Naphthalimide fluorescent probe@MSN	Yes/Yes	2.16–17.3	0.7769	14.8 mg/g	[21]
DNA-functionalized@MSS@ Au NPs	Yes/Yes	0.1079–10.79	0.01079	~99%	[22]
BODIPY-functionalized Fe3O4	Yes/No	0.0002–0.002	220	–	[23]
Non-functionalized Ferrite	Yes/No	0.00002–0.0006	0.000004	–	[24]
Bithiophene-based COF	Yes/Yes	0.02589–10.79	0.02589	398.61 mg/g	[25]
Tetraphenylethene-based fluorescent chemosensor	Yes/No	0–1.4	0.2384	–	[26]

The adsorption kinetics of the three adsorbents was studied by subjecting the data to pseudo 1st and 2nd order models (equations (6), (7))). The fitted kinetic parameters produced R² > 0.99 for the pseudo 2nd order model for all the adsorbents (Fig. 4A–F and Table 7). In addition, the experimental adsorption capacity (Q_{e (Exp.)}) was consistent with the calculated adsorption capacity (Q_{e (Cal.)}) (Table 7). This indicated that all adsorbents followed a pseudo 2nd order model for Ag ⁺ adsorption, which suggested a chemical adsorption behavior. This might be a result of electrostatic interaction between the Ag⁺ ions and the carboxylate groups on the adsorbent surfaces.

Fig. 4.

Pseudo 1st and 2nd order model curves for Ag ⁺ ion adsorption for Gd16-AIS QDs (A–B), Gd20-AIS QDs (C–D), and Gd30-AIS QDs (E–F).

Table 7.

Pseudo 1st order and 2nd order parameters for Ag ⁺ ions adsorption kinetic studies.

	1st order			2nd order
Adsorbent	Qe (Cal)	K1	R2	Qe(Cal.)	Qe (Exp.)	K2	R2
Gd16-AIS QDs	1.341	0.0132	0.0527	476.19	471.98	35	1
Gd20-AIS QDs	2.066	0.0161	0.3188	476.19	472.2	70	1
Gd30-AIS QDs	1.141	0.0031	0.0876	476.19	469.12	35	1

3.4. Adsorption mechanism

The FTIR analysis confirmed the interaction of the Ag⁺ ions with O-H_str (3600-3400 cm⁻¹) and C=O_str (1800-1500 cm⁻¹) groups on the QDs (Fig. 5A). There were increases in the intensities of these functional groups, a slight shift to the lower frequency region, and a small broadening of the band after Ag⁺ ions adsorption. These observations implied feasible electrostatic as well as complexation interactions [27,28]. The PL enhancement of the Gd-AIS QDs after adsorption also supports these observations (i.e., binding of Ag⁺ ion onto the QDs surface), which demonstrated passivated surface defects (Fig. 1A–G). Additionally, the blue-shifted emission suggested some diffusion of Ag⁺ ions into the QD lattice (Fig. 1A–G, Table 2). Furthermore, the electrostatic interaction between the Ag⁺ ions and the adsorbent was supported by the increase in the positive charge on the adsorbent from +4.12 mV to +20 mV at increased Ag ⁺ ion concentrations (Table 8).

Fig. 5.

FTIR spectra of Gd16-AIS QDs before and after the adsorption of Ag⁺ ions (50 ppm) (A), Digital image of Gd16-AIS, Gd20-AIS, Gd30-AIS QDs after Ag⁺ ions adsorption showing the color change from brown (Gd-AIS QDs) to black (Ag⁰ nanoparticles formation) at increased Ag + ions concentrations at increased concentrations (1, 10, 30, 50, 70 and 100 ppm, left to right) (B), and TEM images of Gd16-doped AgInS₂ QDs before (C) and after (D) the adsorption Ag⁺ ions (10 ppm), the proposed mechanism for adsorption of Ag⁺ ions onto the Gd-AIS QDs surface (E).

Table 8.

Zeta potential values of citrate-capped Gd-AIS QDs nanocomposites in the absence and presence of Ag⁺ ions.

	Zeta potential (mV)
Ag+ conc. (ppm)	0	10	30	100
Gd16-AIS@ QDs	+4.16	+5.83	+9.02	+17.6

Higher adsorption capabilities were obtained at lower Gd mole ratios, as a result of the higher number of free carboxylate groups on the QDs surfaces [16]. The pseudo 2nd order model fittings suggested that Ag ⁺ ions removal was related to chemical adsorption processes. These might include electrostatic interactions between the Ag ⁺ ions and the carboxylate groups, followed by the formation of Ag-carboxylate complexes which concurrently could lead to some reduction to Ag nanoparticles formation (Fig. 5B–E). The complexation and reduction reactions were evident from the color change from brown (Gd-AIS QDs) to black (Ag⁰ nanoparticles formation) at increased Ag⁺ ions concentrations (Fig. 5B). The nanoparticle formation was further supported by the results from TEM analysis (Fig. 5C and D), where the initial small spherical shaped QDs of particle size 5.8 nm (Fig. 5C) were surrounded by Ag NPs (Fig. 5D) after their interaction with Ag⁺ ions.

3.5. Industrial wastewater application

The elemental analysis of the real water sample after adsorption showed that both the Ag⁺ and the trace ions (Ti, Pb, Cr, Ni, and Zn) were significantly removed (Table 9). The interaction of the trace elements with the Gd16-AIS QDs was also seen in the PL spectra (Fig. 6). The PL peak of Gd16-AIS QDs was quenched after the adsorption of Ag⁺ ions (47.7 ppm) both in the deionized water and in the water sample (Ag-WS). However, the latter showed a slight increase in the intensity coupled with a shift towards the red region compared to the deionized water after the interaction. This showed some effects of the interfering ions in the Ag-WS. The interaction of ternary I-III-VI QDs with trace concentrations of Ti, Pb, Cr, and Zn through carboxylate binding during has been reported [29]. PL enhancement was reported for interactions with low concentrations of Zn, Cr, and Ti while PL quenching was reported with Pb [13,[30], [31], [32], [33], [34]].

Table 9.

Elemental analysis of acidified industrial wastewater before and after adsorption using Gd16-AIS QDs.

Element	Ag	Ti	Pb	Cr	Ni	Zn
Concentration before adsorption (ppm)	47.7	0.3	0.2	0.6	3.1	2.1
Concentration after adsorption (ppm)	**ND	**ND	**ND	**ND	**ND	**ND
%Ag removal	100	100	100	100	100	100

*ND = Not detected below 0.01 ppm **ND = Not detected below 0.05 ppm.

Fig. 6.

PL spectra of Gd16-AIS QDs alone, with 47.7 ppm Ag⁺ ions (deionized water) and real water (RW) sample. The inset shows the enlargement of the lower parts of the spectra.

These findings revealed that the Gd-AIS QDs could be used for the removal of Ag⁺ ions from industrial wastewater in the presence of trace elements within the respective interference/Ag ratio. Additionally, the material could be used for trace element removal from industrial wastewater. The fluorescence detection of Ag⁺ ions in the Ag-WS was not favorable in the presence of these interfering ions due to the interactions of the trace elements with the QDs that resulted in different PL responses.

4. Conclusion

This work reports the sensing and adsorption kinetics characteristics of Gd-doped AgInS₂ QDs toward detecting and removing Ag ⁺ ions from aqueous solutions. Effects of variation of pH, initial concentration, contact time, and adsorbent dosage were effectively evaluated. All three Gd-doped AIS QDs (Gd-16, 20, and 30) displayed high % removals within 1 min and optimum adsorbent dosage of 0.02 g. Efficient adsorption of Ag⁺ ions was achieved with a pH range of 2–6 and increased Ag⁺ ion concentration resulted in maintained % removals above 95% for all Gd-AIS QDs. The QDs with the least Gd mole ratio (16) exhibited the best sensing characteristics (LOD = 0.88 ppm) as well as adsorption properties (Q _m = 1503 mg/g) and the pseudo 2nd order reaction was well-followed. Although the interaction of the other elements (Ti, Pb, Ni, Cr, and Zn ions) in the industrial wastewaters with the QDs interfered with the fluorescence detection of Ag⁺ ions, the study showed the potential of the Gd-doped AIS QDs in removing Ag⁺ ions from industrial wastewaters without any interference from Ti, Pb, Ni, Cr, and Zn ions.

Author contribution statement

Bambesiwe M. May: Conceived and designed the experiments; Performed the experiments; Analyzed and interpreted the data; Wrote the paper; Contributed reagents, materials, analysis tools or data.

Olayemi J. Fakayode: Conceived and designed the experiments; Analyzed and interpreted the data; Wrote the paper; Contributed reagents, materials, analysis tools or data.

Mokae F. Bambo; Ajay K Mishra; Edward N Nxumalo: Analyzed and interpreted the data; Wrote the paper; Contributed reagents, materials, analysis tools or data.

Data availability

Data will be made available on request.

Declaration of competing interest

The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Bambesiwe M. May reports financial support was provided by National Research Foundation.