Photocatalytic Degradation of Methylene Blue by Surface-Modified SnO₂/Se-Doped QDs

Abstract

Developing new nanomaterials and performing functionalization to increase their photocatalytic capacity are essential in developing low-cost, eco-friendly, and multipurpose-capacity catalysts. In this research, SnO₂/Se-doped quantum dots (QDs) covered with glycerol (SnO₂/Se-GLY) were synthesized using microwave irradiation. Then, their cover was replaced with glutaraldehyde through a ligand exchange procedure (SnO₂/Se-GLUT). The XRD analyses confirmed a tetragonal rutile structure of SnO₂. The HR-TEM analysis confirmed the generation of QDs with a size around 8 nm, and the optical analysis evidenced low bandgap energies of 3.25 and 3.26 eV for the SnO₂/Se-GLY and SnO₂/Se-GLUT QDs, respectively. Zeta-sizer analysis showed that the hydrodynamic sizes for both nanoparticles were around 230 nm (50 mg/L), and the zeta potential confirmed that SnO₂/Se-GLUT QDs were more stable than SnO₂/Se-GLY QDs. The cover-modified QDs (SnO₂/Se-GLUT) showed a higher and faster adsorption capacity, followed by a slower photocatalytic process than the original QDs (SnO₂/Se-GLY). The QTOF-LC-MS analysis confirmed MB degradation through the identification of intermediates such as azure A, azure B, azure C, and phenothiazine. Adsorption isotherm analysis indicated Langmuir model compliance, supporting the high monolayer adsorption capacity and efficiency of these QDs as adsorbent/photocatalytic agents for organic pollutant removal. This dual capability for adsorption and photodegradation, along with the demonstrated reusability, highlights the potential of SnO₂/Se QDs in wastewater treatment and environmental remediation.

1. Introduction

The quantum confinement effect due to the small size of quantum dots (QDs) gives these nanomaterials excellent electrical and optical properties. Cadmium-based QDs are excellent examples because they have a high quantum yield, giving them excellent photocatalytical properties. However, the risk of heavy metal release into the solutions is a threat to environmental applications and increases the risk of human exposure [1]. For this reason, research to find more eco-friendly nanomaterials is indispensable.

SnO₂ nanomaterials have multiple applications, such as gas sensors, batteries, solar cells, and optoelectronic devices [2,3]. SnO₂ bulk material is an n-type semiconductor with a wide bandgap of 3.6 eV corresponding to an absorption around 344 nm [2]. In addition, different SnO₂ nanomaterials have been used to degrade contaminants [4]. For example, core–shell SnSe@SnO₂ has been used to degrade methylene blue [5], the SnO₂-Fe₃O₄ composite degrades Congo red dye [6], and the SnO₂-CuO composite degrades methylene blue [7]. An increase in photodegradation capacity using dopants has been reported; for example, Mn and Zn-doped SnO₂ nanoparticles degrade methylene blue [8,9].

The difference between all these nanomaterials is the synthesis or generation of the nanocomposites. The different synthesis methods include hydrothermal, chemical vapor deposition, electrospinning, lithography, etc. [10]. Microwave-assisted synthesis has also been used to synthesize nanomaterials like CdS/Se QDs [11], PbS nanostructures [12], and metal nanoparticles [13]. The advantages of the use of microwave irradiation include homogeneous heating, the generation of nanomaterials in a short time, the use of non-toxic solvents, the production of fewer waste products, etc.

Based on the above-mentioned reasons, this research studied the adsorption and photocatalytic capacity of SnO₂/Se-doped QDs synthesized by microwave irradiation and covered with glycerol (SnO₂/Se-GLY) and glutaraldehyde (SnO₂/Se-GLUT) after a ligand exchange methodology. Although the adsorption and photocatalytic capacities of different nanomaterials have been reported, this is the first time that both processes have occurred at the same time using SnO₂/Se-doped QDs. In addition, the increase in the adsorption capacity of SnO₂/Se QDs (without losing photocatalytic capacity) by modifying their cover is reported for the first time.

2. Materials and Methods

2.1. Synthesis of SnO₂/Se-GLY QDs

To synthesize the SnO₂/Se-GLY QDs, a 5% glycerol (GLY) water solution was mixed with a SnCl₂ solution at pH 11.5 to generate GLY-Sn. Then, the pH was lowered to 6.2–6.5 with HCl (1.0 and 0.1 M), and 50 µL of fresh reduced selenide solution (0.1 M) was added (selenium powder was reduced using sodium sulfite in a reflux system for 8 to 10 h). The pH solution was adjusted to 6.2–6.5, and the mixture was transferred to a Teflon™ vessel (CEM Corporation, Matthews, NC, USA). The vessels were placed in a MARS 6 Microwave system (CEM Corporation, Matthews, NC, USA), and the synthesis was performed at 130 °C with a 15 min ramp and 10 min reaction time. The suspension containing the nanoparticles was precipitated using 2-propanol, and the nanomaterial was recovered by centrifugation. The SnO₂/Se-doped nanoparticles covered with GLY (SnO₂/Se-GLY) were resuspended and stored in deionized water.

2.2. Preparation of SnO₂/Se-GLUT QDs

A ligand exchange method was used to prepare SnO₂/Se-GLUT QDs [11]. In a 50 mL centrifuge tube, SnO₂/Se-GLY nanoparticles were mixed with glutaraldehyde. Phosphate-buffered saline (PBS) was used to stabilize the pH at 7. The final concentrations of the nanoparticles and glutaraldehyde were 1000 mg/L and 2.5%, respectively. The suspension was mixed using a Rotamix™ (ATR Biotech, Laurel, MD, USA) at 30 rpm for 24 h. The SnO₂/Se nanoparticles covered with glutaraldehyde (SnO₂/Se-GLUT) were recovered by precipitation with 2-propanol and centrifuged. The SnO₂/Se-GLUT nanoparticles were resuspended and stored in deionized water.

2.3. Characterization of SnO₂/Se-GLY and SnO₂/Se-GLUT QDs

The optical characterization (absorption and emission) of both nanomaterials was performed using a UV-VIS 1800 spectrophotometer (Shimadzu, Columbia, MD, USA) and a spectrofluorometer RF 6000 (Shimadzu, Columbia, MD, USA). The presence of glycerol and glutaraldehyde on the surface of the nanoparticles was analyzed with an FT-IR Cary 630 (Agilent Technologies, Santa Clara, CA, USA). The crystal pattern of the nanoparticles was analyzed using a SmartLab X-ray diffractometer (Rigaku, Woodlands, TX, USA). The nanometric size of the nanomaterial was evaluated using high-resolution transmission electron microscopy (HRTEM) on a JEM-ARM200CF (ThermoFisher Scientific, JEOL, Tokyo, Japan). The hydrodynamic size and the stabilization of the nanomaterials were analyzed using a Zetasizer Pro Blue (Malvern Panalytical, Worcestershire, UK).

2.4. Batch Adsorption/Photodegradation Experiments of Methylene Blue

For the batch experiments, 20 mL solutions at low (5 y 10 µM) and high concentrations (50 and 100 µM) were contacted with the SnO₂/Se-GLY and SnO₂/Se-GLUT QDs at a concentration of 200 mg/L (0.2 g/L) in 50 mL centrifuge plastic tubes. The tubes were mixed with a Rotamixer at 30 rpm. In order to evaluate the adsorption of methylene blue on the nanoparticles, the solutions were placed in the dark, and samples were taken at specific times until the residual concentration in the solution reached an equilibrium. After 18 h in the dark, the solutions were irradiated in a PVC box with two visible lamps, each with 55 V and 5000 lumens of output. Samples were taken at specific times, and the residual concentration of methylene blue was monitored for seven additional hours. The residual concentrations of the solution were analyzed in an Ultimate 3000 Ultra High-Performance Liquid Chromatographer (Thermo Fisher Scientific, Waltham, MA, USA), monitoring the absorbance at 600 nm. All batch experiments were carried out in duplicates. The photodegradation products that absorb at the same wavelength were evaluated during the HPLC analysis, and the molecular structures were determined using a 6530 accurate-mass QTOF-LC-MS (Agilent, Santa Clara, CA, USA).

2.5. Recycling Study of the SnO₂/Se-GLY and SnO₂/Se-GLUT QDs at Low Concentrations of Methylene Blue

The reuse capability of SnO₂/Se-GLY and SnO₂/Se-GLUT nanoparticles was evaluated for four consecutive batch cycles. For each cycle, methylene blue solution at 5 and 10 µM concentrations was contacted with 200 mg/L of each nanoparticle. The absorption process was evaluated in the dark during the first six hours, and the photodegradation process was evaluated during the next 2 h. After each batch, the solutions were centrifuged, and the pellets containing the nanoparticles were resuspended in fresh methylene blue at the corresponding concentrations.

3. Results

3.1. Synthesis and Functionalization of SnO₂/Se QDs

The microwave synthesis gives water-stable SnO₂/Se-GLY QDs with a salmon color (Figure 1, left), and the color is maintained after the ligand exchange (Figure 1, right).

3.2. SnO₂/Se-GLY and SnO₂/Se-GLUT Characterization

Figure 2 shows the optical properties of the SnO₂/Se-GLY nanoparticles. The UV-Vis absorption analysis shows that the light absorption starts in the range around 800 nm, confirming the capacity of the nanoparticles to absorb energy in the visible range. A high direct bandgap energy of 3.25 eV (inset Figure 2) corresponding to 382 nm was observed. This bandgap energy is lower than that of the standard material (3.6 eV). Some authors have reported low values of 3.05 eV [6] and 3.16 eV [14].

Figure 3 shows the absorption spectrum of the SnO₂/Se-GLUT QDs, where a slight decrease in the visible region is observed. The calculated bandgap energy is 3.26 eV, which corresponds to 380 nm. This small change in the absorption indicates small changes on the QD surface.

Figure 4 shows the functional groups of the covers (glycerol and glutaraldehyde) and the residues of the cover on the QD surface. The SnO₂/Se-GLY QD spectrum obtained after microwave synthesis shows some characteristic signals of pure glycerol, such as the O-H around 3200 cm⁻¹, confirming the presence of the organic cover on the QD surface. The shift in the C-O bonds of glycerol on the SnO₂/Se-GLY QD surface (compared with pure glycerol) suggests a possible interaction of glycerol with the QDs through the C-O functional groups. The differences between the SnO₂/Se-GLY and the SnO₂/Se-GLUT spectra confirm the ligand exchange of glycerol by glutaraldehyde, for example, the disappearance of a C-O signal around 1360 cm⁻¹. Changes in the IR spectra were observed after the ligand exchange procedure with other organic covers on the nanoparticle’s surface [11].

The synthesized Se-doped SnO₂ QDs, stabilized with glycerol and glutaraldehyde, exhibit a tetragonal rutile structure consistent with the cassiterite phase (JCPDS No. 41-1445), as shown by the diffraction planes (110), (101), (200), (211), (220), and (301) at respective 2θ angles of 26.38°, 33.93°, 38.00°, 51.77°, 54.71°, and 66.04° (Figure 5). These findings confirm SnO₂ as the primary crystalline phase. Evidence of selenium doping is suggested by a shift toward higher diffraction angles for key planes, attributed to the substitution of Sn⁴⁺ ions with smaller Se⁴⁺ ions (ionic radius of 0.50 Å vs. 0.69 Å for Sn⁴⁺). This ionic substitution leads to a contraction of the lattice, resulting in decreased interplanar spacing and, consequently, a shift in diffraction peaks to higher angles as per Bragg’s law. Notably, the (101), (200), and (301) planes exhibit angles of 33.93°, 38.00°, and 66.04°, respectively, compared to reference values of 33.70°, 37.86°, and 65.82°, further confirming the impact of selenium incorporation on the crystal structure.

Lattice parameters are influenced by several factors, including defects, external strain, dopant concentration, and the difference in ionic radii between host and dopant ions. In this study, the lattice parameter of the doped SnO₂ particles was calculated using Bragg’s law and the diffraction angles, yielding a value of 3.732 Å. This value is significantly smaller than the lattice parameter of pure SnO₂, which is 4.738 Å. The observed reduction in the lattice parameter confirms that selenium ions (Se⁴⁺) have substituted tin ions (Sn⁴⁺) in the SnO₂ crystal lattice. The smaller ionic radius of Se⁴⁺ (0.50 Å) compared to Sn⁴⁺ (0.69 Å) leads to a contraction in the crystal structure, confirming the incorporation of selenium into the lattice [15,16].

The presence of selenium as a dopant is essential in facilitating the controlled formation of SnO₂/Se-doped nanoparticles. In the absence of selenium, nanoparticle formation does not occur. Additionally, when synthesis conditions exceed 130 °C or extend beyond a reaction time of 10 min, the process yields a black SnO₂ solution, indicative of bulk SnO₂ crystallization rather than nanoscale formation. This same result is observed if residual SnO₂ crystals are present from previous syntheses in the reaction vessel, which act as nucleation sites for uncontrolled crystal growth.

The nanometric size of SnO₂/Se-GLY was confirmed using HR-TEM (Figure 6a). The crystal size is around 8 nm with a crystal lattice of 0.33 nm (zooming in Figure 6), which confirms the generation of SnO₂ quantum dots doped with selenium. Crystals of similar sizes have been reported in the literature [1]. The electron diffraction analysis (Figure 6b) provides strong evidence for the presence of a tetragonal crystal structure in the synthesized material. This conclusion is supported by the observation of the characteristic Miller indices (110), (101), (200), and (211), which are typical for a tetragonal lattice. The electron diffraction pattern of the SnO₂/Se-GLY QDs also suggests the generation of a monocrystalline nanomaterial.

The stability and hydrodynamic size of the nanomaterials are essential for specific applications. The dispersion of the nanoparticles in water was analyzed at two different concentrations (50 and 500 mg/L) and before and after 5 min of sonication in a water bath (Figure 7). The size variation after sonication of the SnO₂/Se-GLY QDs suggests that the storage generates aggregation of the QDs (Figure 7a); however, sonication at low concentrations is enough to disperse the nanomaterial. The aggregation is so strong at high concentrations (500 mg/L) that sonication cannot release the nanoparticles. The hydrodynamic size of the SnO₂/Se-GLUT QDs is more stable at 50 and 500 mg/L (Figure 7b). These results suggest that the hydrodynamic size of both nanoparticles is between 230 and 250 nm. The zeta potential values of the SnO₂/Se-GLY and SnO₂/Se-GLUT QDs are −20.20 and −29.78 mV, respectively. These results suggest that both nanoparticles tend to generate agglomerates (values lower than −60 and 60 mV), and the SnO₂/Se-GLUT QDs are more stable (value more negative).

3.3. Adsorption and Photodegradation of Methylene Blue Using SnO₂/Se-GLY and SnO₂/Se-GLUT QDs

During the first hours in the dark, the concentration of methylene blue decreases around 39% and 20% for the initial concentrations of 5 and 10 µM (respectively) using 200 mg/L of the SnO₂/Se-GLY, indicating that methylene blue is adsorbed on the QD surface (Figure 8). The same adsorption pattern was observed for the SnO₂/Se-GLUT QDs; the initial concentrations of 5 and 10 µM of methylene blue decreased by around 77% and 83%, respectively. It is evident that the cover modification of the original SnO₂/Se-GLY QDs with glutaraldehyde increased their adsorption capacity. These findings could be attributed to a stronger interaction between the carbonyl group of the glutaraldehyde and the amine tertiary of methylene blue compared to the interaction between the alcohol of glycerol and the amine tertiary of methylene blue. In both cases, it is well known that the reaction of a carbonyl or alcohol group with an amine tertiary does not yield a stable product.

After the adsorption process was completed, the solutions were irradiated with visible light, and photodegradation of methylene blue was observed (Figure 8). Although the photodegradation process gave 100% methylene blue elimination for all experiments, the process was faster when SnO₂/Se-GLUT was used. These results corroborate the fact that methylene blue must be located near the QDs to be degraded by the reactive oxygen species or directly degraded by the electron/hole pair.

At high concentrations of methylene blue, it was not adsorbed on the SnO₂/Se-GLY QDs in the dark after 18 h (Figure 9). In contrast, at 50 and 100 µM of methylene blue, the adsorption onto SnO₂/Se-GLUT was 33% and 55%, respectively, confirming the same pattern as at low concentrations because of the presence of glutaraldehyde on the QD surface.

After adsorption, the photocatalytic process by both nanoparticles is evident. Due to the high concentrations, photodegradation takes longer than at low concentrations. SnO₂/Se-GLY QDs at 200 mg/L completely degraded methylene blue during the first 2 and 5 h for 50 and 100 mg/L, respectively. SnO₂/Se-GLUT QDs at 200 mg/L completely degraded 50 µM of methylene blue in 5 h, and after 7 h, 95% of 100 µM of methylene blue solution was degraded, but the tendency was to degrade it completely. These results confirm the capacity of the SnO₂/Se-GLY and SnO₂/Se-GLUT QDs to be used as adsorbent/photocatalytic agents to remove environmental contaminants using adsorption and photocatalytic processes. The adsorption capacity of nanoparticles has been reported in the literature. For example, carbon nanotubes have been used to adsorb organic dyes [13]. SnO₂ nanoparticles [17] and SnO₂ QDs doped with nickel [18] have been used to photodegrade methylene blue under visible light, achieving high photodegradation percentages of over 90%.

3.4. Adsorption Isotherm of Methylene Blue Using SnO₂/Se-GLUT QDs

The photocatalytic process was confirmed using QTOF-LC-MS analysis. The chromatograms during the first 18 h in the dark only show the presence of two peaks corresponding to methylene blue (principal peak) and a small peak corresponding to azure B, which is a degradation of methylene blue during storage. No other peaks were observed. In contrast, after the first 15 or 30 min of visible irradiation, more peaks were observed, confirming the photodegradation of methylene blue and azure B. These peaks increased and changed with the irradiation time. The QTOF-LC analysis confirmed the presence of azure B (271 m/z MH + 1), azure A (257 m/z MH + 1), azure C (243 m/z MH + 1), and phenothiazine (201 m/z MH + 1). The same photodegradation pattern was observed for methylene blue degraded with other nanoparticles [19].

The adsorption process of methylene blue was only evaluated for the SnO₂/Se-GLUT QDs because they show adsorption at low and high concentrations. The adsorption results were analyzed using the isotherms of Langmuir and Freundlich. For this analysis, additional solutions were placed in contact with both nanoparticles in the dark to obtain reliable points in the plot. The Langmuir isotherm assumes that methylene blue is adsorbed on the QD surface in identical binding sites, generating a monolayer [20]. The linear form of the Langmuir model (Lineweaver–Burk) is expressed as

$${\displaystyle \frac{1}{q_e}}={\displaystyle \frac{1}{q_m}}+{\displaystyle \frac{1}{q_m{K}_L}}{\displaystyle \frac{1}{C_e}}$$

where q_e is the amount of methylene blue adsorbed per unit mass of the QDs (mg/g), q_m is the maximum adsorption capacity of the monolayer, K_L is the equilibrium parameter between adsorption and desorption processes, and C_e is the equilibrium concentration of methylene blue (mg/L) in the solution.

The Freundlich isotherm assumes that the surface of the QDs is heterogeneous and methylene blue is adsorbed on binding sites with an exponential function. The linear form of the Freundlich model is expressed as

$$\log q_e=\log K_f+{\displaystyle \frac{1}{n}}\log C_e$$

where q_e is the amount of methylene blue adsorbed per unit mass of the QDs (mg/g). K_f and 1/n are the Freundlich constants that are characteristics of the system. Ce is the solution’s equilibrium concentration of methylene blue (mg/L). Figure 10a shows that the adsorption of methylene blue on the SnO₂/Se-GLUT QDs follows an adsorption isotherm (type I) where methylene blue is adsorbed on the nanoparticle, forming a monolayer. The results were evaluated using a Langmuir and Freundlich isotherm. The best fitting (r² = 0.997) was obtained using the Langmuir isotherm (Figure 10b), which could be explained by the nanomaterial’s small size. According to the Langmuir model, it generates identical specific binding sites that allow the formation of a methylene blue monolayer on the nanoparticle surface [20]. The adsorption rate was 0.285 1/mg, and the maximum capacity (qm) was 94.3 mg/g (

Table 1. Adsorption isotherm parameter of methylene blue adsorption on the SnO₂/Se-GLUT QDs.

QDs	Langmuir Isotherm Parameters			Freundlich Isotherm Parameters
	KL (1/mg)	qm (mg/g)	R2	KF (1/mg)	1/n	R2
SnO2/Se-GLUT	0.285	94.3	0.997	17.0	0.608	0.926

), indicating a high adsorption capacity by the SnO₂/Se-GLUT QDs due to their small crystal size. Authors have reported similar qm values for the removal of methylene blue using lignin nanoparticles (127.91 mg/g) [21], SiO₂ nanoparticles (70.16 mg/g) [22], and SnO₂ nanorods (7.32 mg/g) [23].

The Freundlich model gave a low fitting of 0.926. The 1/n parameter (0.608) from the Freundlich isotherm (Table 1) confirms methylene’s strength and favorable adsorption on the SnO₂/Se-GLUT QDs. In addition, a change in the color of the methylene blue solution was observed after adding the SnO₂/Se-GLUT QDs, which suggests a fast interaction of methylene blue with the QDs. After centrifugation, the adsorption of methylene blue on the SnO₂/Se-GLUT QDs is evident (inset in Figure 10a). The change in the color of methylene blue to violet has been related to the change in the counter ion in the solid state that confirms the proximity (adsorption) of methylene blue to the SnO₂/Se-GLUT QDs.

3.5. Kinetic Analysis of Methylene Blue Photodegradation Using SnO₂/Se-GLY and SnO₂/Se-GLUT QDs

For the photodegradation process (after adsorption), the data were analyzed to determine the kinetic order. First- and second-order kinetic models were used. The linear model of the first-order kinetic model is expressed as

$$ln\left[A\right]=ln{\left[A\right]}_0-Kt$$

The linear second-order kinetic model is expressed as

$${\displaystyle \frac{1}{\left[A\right]}}={\displaystyle \frac{1}{{\left[A\right]}_0}}+Kt$$

where [A] is the concentration during the experiment and [A]₀ is the initial concentration of methylene blue, K is the rate constant, and t is time. The parameters were calculated for methylene blue degradation at low and high SnO₂/Se-GLY and SnO₂/Se-GLUT QD concentrations (

Table 2. First- and second-order kinetic parameters of methylene blue photodegradation using SnO₂/Se-GLY and SnO₂/Se-GLUT QDs.

QDs (MB Concentration)	First-Order Kinetic Model		Second-Order Kinetic Model
	R2	K	R2	K
SnO2/Se-GLY (5 and 10 µM)	0.96 ± 0.06	2.65 ± 1.11	0.91 ± 0.05	2.71 ± 1.31
SnO2/Se-GLY (50 and 100 µM)	0.95 ± 0.07	0.92 ± 0.13	0.85 ± 0.14	0.39 ± 0.09
SnO2/Se-GLUT (5 and 10 µM)	-	-	-	-
SnO2/Se-GLUT (50 and 100 µM)	0.98 ± 0.02	0.47 ± 0.21	0.90 ± 0.04	0.24 ± 0.31

). The results suggest that the photodegradation of methylene blue using both QDs follows a pseudo-first-order kinetic model (highest R²). These results indicate that the degradation depends on the methylene blue concentration, and the QDs are the photocatalytic agents. The half-life time for the first-order kinetic model can be calculated using the following equation:

$$t_{{\displaystyle \raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$2$}\right.}}={\displaystyle \frac{0.693}{K}}$$

The half-life (t½) times calculated are 0.36 and 1.05 h for SnO₂/Se-GLY at low and high concentrations, respectively. These results indicate that the degradation is faster at low concentrations of methylene blue. The half-life time for SnO₂/Se-GLUT at high concentrations is 2.06 h, indicating that the degradation of methylene blue using SnO₂/Se-GLUT QDs is slower than that using SnO₂/Se-GLY QDs.

3.6. Recycling Study of SnO₂/Se-GLY and SnO₂/Se-GLUT QDs

Reusing the QDs or other nanomaterials is essential to ensure low-cost and eco-friendly catalysts. For each recycle cycle, the solutions were centrifuged, recovered, and placed in contact with a fresh 5 µM solution of methylene blue. Because Figure 6 and Figure 7 indicate that the adsorption process is during the first hours, a 4 h step in the dark to allow the adsorption of methylene blue was included in each cycle, followed by irradiation. It is evident that the adsorption of methylene blue decreased during the batches, but methylene blue was completely photodegraded (Figure 11). Different authors have proved the recycling capacity of metal oxide and selenide nanomaterials [24,25,26].

4. Conclusions

This study explores the absorption and photocatalysis of methylene blue dye in the presence of visible light-activated SeO₂/Sn QDs with modified surfaces. The results indicate that SnO₂/Se-GLUT QDs evidenced a higher and faster adsorption capacity, followed by a slower photocatalytic process, compared to the original QDs (SnO₂/Se-GLY). Furthermore, both types of QDs (GLU- and GLY-modified QDs) can be reused multiple times, and they adsorb methylene blue according to the Langmuir adsorption isotherm and photodegrade it following a pseudo-first-order kinetic model. The results suggest the potential application of these dual-function QDs (for both absorption and photocatalytic purposes) in treating water contaminated with organic dyes.