Silver nanoparticles obtained using cyclodextrin derivatives and pectin as a key component of titanium dioxide-based composites for water purification

Abstract

This study investigated the effect of β-cyclodextrin (β-CD), its derivatives, and pectin on reducing Ag⁺ ions for the synthesis of silver nanoparticles (AgNPs) and corresponding composites with TiO₂ (TiO₂/Ag), which were evaluated as high-performance photocatalysts for degrading methyl orange (MO) in both model solutions and natural water samples. AgNPs were obtained at a significantly lower molar ratio of Ag:β-CD (9:1) and in the pH range from 5.5 to 7.5. The influence of cyclodextrins on the reduction of Ag⁺ ions to AgNPs in water and on the surface of titanium dioxide was researched. The average particle size of TiO₂/Ag determined from SEM data ranged from 125 nm to 140 nm, depending on the type of reducing agent. When pectin was used, the samples showed less aggregation, with an average particle size of 125–127 nm. The photocatalytic degradation process was monitored by UV-Vis spectroscopy in different types of water media and pH. The best photodegradation results for MO were achieved with samples prepared from silver methacrylate. In this case, complete decolorization of the MO solution occurred within 30 min at pH 2.5 and 70 min at pH 7.5, which is 2.5 times and 3 times faster than with pure TiO₂.

Supplementary Information

The online version contains supplementary material available at 10.1038/s41598-026-43099-2.

The pollution of natural water bodies caused by industry has been an urgent issue for many decades. Synthetic dyes are among the most common organic pollutants due to their widespread use in the textile, printing, leather, paper, cosmetics, food, and pharmaceutical industries. These dyes are toxic chemicals that damage the environment and human health¹. The inefficiency of industrial treatment facilities makes water unsafe for drinking, irrigation, and cooling systems. Currently, many physical and chemical methods are used for wastewater treatment, including adsorption, reverse osmosis, precipitation, chemical oxidation, ozonation, membrane filtration, and ion exchange². Although adsorption is the most common, effective, and promising method¹, photocatalytic degradation for water purification avoids some issues inherent to other methods, in particular the use of additional reagents and the formation of secondary waste, while increasing the efficiency of removing persistent organic compounds. Among photoactive materials, TiO₂ is of particular interest due to its chemical stability, low cost, availability, non-toxicity, and presence of favorable electronic and optical properties. The photocatalytic efficiency of TiO₂ is limited by its UV-only activation and high electron-hole recombination rate³. Furthermore, nanoparticle agglomeration and poor organic molecule adsorption hinder its practical use in purification systems^4–8. However, its large band gap (3.2 eV) and high charge carrier recombination rate under visible light make it an ineffective photocatalyst^9–13. To reduce the band gap and increase visible light absorption and photocatalytic efficiency, TiO₂ is modified with various compounds, including cyclodextrins¹⁴. Cyclodextrins (CD) are cyclic oligosaccharides consisting of a hydrophobic inner cavity and a hydrophilic outer surface. This combination results in satisfactory flotation properties of CD in aqueous solutions on the one hand, and on the other hand, ensures effective interaction with organic compounds, capturing and retaining them as a guest-host interactions. In addition, β-CD also play an important role in photogenerated charge transfer, as they can serve as hole traps to suppress electron-hole pair recombination^15–17. However, with an increase in β-CD content, the efficiency of destruction, for example, of methylene blue, increased more slowly¹⁸. It is known that the overall photocatalytic activity of catalysts can be significantly increased by doping plasmonic metals such as Ag, Au, Cu, etc., on binary composites based on semiconductors^19,20. These metals excite simultaneous oscillations of conduction electrons with a resonant frequency, which is called the localized surface plasmon resonance (SPR) effect, significantly increasing the absorption of visible light^21–23. The synergistic effect of Ag and β-CD has been noted in numerous studies, and TiO₂/Ag/β-cyclodextrin hybrid nanocomposites have been successfully applied in the field of ecology to study their photocatalytic activity in visible light during the decomposition of methylene blue^18,24, for the formation of electrospun floating photocatalytic membranes for wastewater treatment²⁵, and for the highly efficient decomposition of naphthalene²⁶. In agricultural engineering, it is used to increase soil fertility through enhanced photocatalytic oxidation of urea under the action of sunlight^27,28; in the food industry, it boosts antibacterial activity and extend the shelf life of products²⁹, in the automotive industry – to improve fuel characteristics by deep desulfurization of gasoline³⁰, research on the stability and thermophysical properties of functionalized β-CD-TiO₂-Ag nanoliquids for antifreeze³¹.

Various physical, chemical, and biological methods are utilized to synthesize silver nanoparticles, including laser ablation³², liquid plasma³³, sputter deposition³⁴, microwave³⁵, sonochemical³⁶, photochemical reduction^37,38, electrochemical reduction³⁹, radiation-chemical reduction⁴⁰, thermochemical reduction^41–43, chemical reduction^44–48 and green approaches^49,50. In chemical reduction, sodium borohydride^44,45, hydrazine^46,47, and ascorbic acid^47,48 are most commonly exploited as reducing agents. In thermochemical reduction, chitosan, polyethyleneimine, pectin, and cyclodextrin were involved as reducing agents in polyelectrolyte complexes⁴¹ or separately introduced into the polymer matrix. The size of silver nanoparticles obtained by thermochemical reduction was 4–9 nm.

Modern methods for creating metal-containing nanomaterials increasingly use natural polymers, which serve as both reducing agents and stabilizers. Pectin, a polysaccharide rich in carboxyl and hydroxyl groups, can reduce the amount of Ag⁺ in an alkaline environment, where the reducing hemiacetal hydroxyl groups of pectin are activated. Together with secondary hydroxyl groups of arabinan, galactan and fragments of D-galacturonic acid in pectin, they participate in the reduction reaction, oxidizing to ketones. Pectin also forms a strong stabilizing layer on particle surfaces, preventing aggregation and enabling colloids with a well-controlled size of approximately 8–14 nm to be produced^51–55.

In turn, cyclodextrins (in particular β-cyclodextrin) can also act as mild reducing agents and effective stabilizers due to their numerous hydroxyl groups and hydrophobic cavity. Their application ensures the formation of nanoparticles with low dispersibility and high stability. Typically, reduction is carried out without additional reducing agents in an alkaline environment (pH > 9) in the presence of CD and pectin. However, classic reducing agents – borohydride^56–58, hydrazine⁵⁹, glucose^60–62, and sodium citrate⁶³ are sometimes also involved in combination with CDs, with the size of the nanoparticles formed ranging from 6.3 nm⁵⁸ to 50 nm⁶¹. However, NaOH is most often used to activate the reduction reaction of silver ions. A fairly wide range of Ag⁺: β-CD molar ratios (from 1:175 to 3:1, most often 3:1 to 1:3^35,64–66, temperatures (from 4 to 200 °C), and pH levels (from 7, more often 9, to 12) are used to synthesize silver nanoparticles. A number of studies have shown that the average size of silver nanoparticles ranges from 5 to 10 nm^35,58,67,68, with an SPR band at 398–402 nm and a molar ratio of Ag⁺:β-CD from 1:2 to 1:175. Iacovita et al.³⁵ investigated the synthesis of silver and gold nanoparticles with β-CD as a simultaneous reducing agent and stabilizer, under microwave (MW) and ultrasound conditions. The authors showed that changing the alkaline medium (replacing NaOH with K₂CO₃) doubles the size of AgNPs in MW synthesis (from 5 to 10 nm to 15–20 nm). Under ultrasound, the particles are less stable, with further changes observed after treatment, whereas synthesis at room temperature in the presence of β-CD and K₂CO₃ yields stable AgNPs (~ 20 nm) and AuNPs (~ 11 nm). The work shows that the size of silver nanoparticles significantly depends on the synthesis conditions and the amount of β-cyclodextrin: under microwave exposure and in an alkaline environment using NaOH, very fine particles (< 10 nm) were formed, while replacing the base with K₂CO₃, and maintaining the reagent ratio led to an increase in size to ≈ 20 nm. For synthesis at room temperature, an Ag⁺: β-CD ratio of ~ 1:20 provided stable particles of about 20 nm. The plasmon absorption band of AgNPs in all approaches remained in the range of ≈ 398–404 nm, corresponding to small spherical particles and reflecting the peculiarity of β-CD stabilization, which inhibits a red shift even with an increase in core size. All the particles obtained exhibit surface-enhanced Raman scattering (SERS) activity, enabling the detection of hydrophobic molecules (e.g., naphthalene, melamine) and even the differentiation of propranolol enantiomers. In addition, β-CD capping reduces the pH-dependent influence on SERS spectra and alters the spectral properties of the analyzed molecules, underscoring the role of cyclodextrin as a functional macrocycle in plasmonic nanosystems.

Cyclodextrins play a key role in stabilizing silver nanoparticles by adsorbing onto their surfaces through hydroxyl groups, forming a protective shell and providing sites for specific interactions with various analytes. As a result of these interactions, a guest-host complex forms, leading to a decrease in the intensity of the silver SPR band and the appearance of a new band in the long-wave range (500–700 nm) due to nanoparticle aggregation, which is used for the quantitative determination of the analyte. Such colorimetric sensors have been developed for melamine⁶⁷, zidovudine⁶⁶, tetrahydrocannabinol⁶², riboflavin⁶⁹, and hydrogen peroxide in urine⁷⁰. When detecting Hg²⁺, S^2-71, and Ni²⁺⁷², a decrease in the intensity of the SPR band was observed due to the chemical interaction of silver with these ions. To identify analytes such as marbofloxacin⁷², thiacloprid and imidacloprid⁷³, norfloxacin⁷⁴, sulfamethazine and levofloxacin⁶⁸ surface-enhanced Raman spectroscopy (SERS) was employed using a β-CD functionalized silver nanoparticles substrate.

In this study, the impact of β-CD, its derivatives, and pectin as natural component on the reduction of Ag⁺ ions to the synthesis of AgNPs and corresponding composites with TiO₂ (TiO₂/Ag) was investigated. The main focus is primarily on studying the peculiarities of obtaining silver nanoparticles with a significantly lower molar ratio of Ag:β-CD (9:1) and a lower pH range from 5.5 to 7.5, which has not yet been described in the literature. The physical-chemical properties of starting and resulting samples were characterized using X-ray diffraction (XRD, for crystal structure), dynamic light scattering (DLS, for particles size), Fourier transform infrared spectroscopy (FTIR), and ultraviolet-visible spectroscopy (UV–Vis). The photocatalytic activity of the synthesized catalysts was investigated for the treatment of organic dye (i.e. methyl orange (MO)) under UV irradiation. Thus, the aims of the study were to (a) optimize synthesis of TiO₂/Ag catalysts at different operating parameters such as source of Ag⁺ ions for chemical reduction, time and temperature, type of reducing agent including β-CD, etc., (b) provide the degradation of MO under UV irradiation, (c) investigate the degradation of MO by various photocatalysts and compared the results obtained from neat TiO₂ and TiO₂/Ag, (d) research kinetics of MO degradation, and finally (e) conduct degradation of MO in model solution and environmental water.

Experimental

Materials

β-cyclodextrin (β-CD) (Cavamax W7, Wacker), 2-hydroxypropyl-β-cyclodextrin (HP-CD) (DS = 4.5) (Cavasol W7, Wacker) were produced by Wacker Germany.

Methyl-β-cyclodextrin (DS = 12) (Ме-CD), sulfobutyl ether sodium salt β-cyclodextrin (DS 6.5) (SB-CD), Carboxymethyl-β-cyclodextrin (CM-CD) (DS = 3.5), CD-bead polymer were purchased from Cyclolab (Hungary). Maleoyl-β-cyclodextrin was synthesized as described in⁷⁵.

Titanium dioxide (TiO₂), silver nitrate (AgNO₃), silver nanoparticles (AgNPs, < 100 nm), sodium borohydride, methacrylic acid were supplied by Sigma-Aldrich (Germany). Pectin with degree of esterification 77.8%, was supplied by Cargill Deutschland GmbH (Germany).

Instrumental methods

Electronic spectroscopy. UV-Vis spectra were recorded using a UV-2401 PC spectrophotometer (Shimadzu, Japan) in the wavelength range of 190–800 nm. Distilled water was used as the reference solution to take the background line for all measurements.

Fourier Transform Infrared (FTIR) spectroscopy. FTIR spectra were obtained using a Tensor-37 spectrometer (Bruker, Germany) in the range 400–4000 cm^–1. These measurements were evaluated by making pellets from KBr powder, which acts as a spectroscopically transparent diluent.

X-ray diffraction (XRD) analysis. Powder XRD data of the photocatalysts was recorded by XRD-7000 diffractometer (Shimadzu, Japan) using CuK_α radiation.

The average crystallite size (D, nm) and the interplanar spacing (d, Å) of solids were calculated using the Scherrer⁷⁶ and Bragg equations, respectively, as follows:

where К is a constant depending on the particle morphology of scattering object and varies from 0.89 to 1.39 (i.e. for particles of unknown shape – 0.9, for spherical particles – 0.94); β is the full width at half maximum (FWHM) of the peak; θ is the Bragg angle; λ is the wavelength of characteristic X-ray radiation (λ = 1.54 Å for СuKα radiation).

Lattice parameters of solids could be calculated with the formula as follows:

1/d² = (h² + k²)/a² + l²/c² = a²c.

where d is the interplanar spacing; h, k and l are the Miller’s indices; a and c are the lattice parameters; V is the cell volume.

The morphology of the samples was studied using a JEM-1230 transmission electron microscope (JEOL, Japan) and a JEOL JSM-5600 scanning electron microscope at high vacuum (HV) and Secondary Electron Image (SEI) mode. The chemical composition of the nanocomposites was analyzed by energy-dispersive X-ray spectroscopy (EDX, FEI Quanta FEG 660) operating at 20 kV. Particle diameter measurements, data processing, and histogram creation were performed using ImageJ and OriginPro 8.

For the quantitative determination of silver in TiO₂/Ag, the sample (0.1000 g) was suspended in 10 ml of dilute HNO₃ (1 : 1) and CO₂-free distilled water (total 50 mL) and analyzed in duplicate by atomic absorption spectroscopy by C-115M1 aparatus at 328.1 nm.

Dynamic light scattering (DLS) analysis. The particle size of each sample was measured in triplicate at 25 °C by dynamic light scattering using a Litesizer 500 (Anton Paar, Austria) (40 mW red semiconductor laser of 658 nm) at 175° scattering angle. The TiO₂ samples were ultrasonically dispersed in water at a concentration of 1 mg/ml for 30 min prior to testing. The size measurements of each sample (~ 1 ml per sample) were presented in terms of three metrics, including intensity-based size, number-based size and volume-based size. The zeta potential was determined using the Smoluchowski approximation from electrophoretic mobility measurements based on Henry’s equation. The instrument automatically set the measuring angle to 15° within the range from − 600 mV to + 600 mV.

Preparation of Ag₃Cit and AgC₄H₆O₂

Silver citrate (Ag₃Cit) and, similarly, methacrylate (AgC₄H₆O₂) was synthesized by precipitation from the reaction between AgNO₃ and sodium citrate at a molar ratio of 3:1 or 1:1⁷⁵. This reaction resulted in the formation of a white precipitate, which was filtered, washed with distilled water, and dried at 50 °C away from light. Silver methacrylate (AgC₄H₆O₂) was synthesized at the same conditions using AgNO₃ and methacrylic acid solutions.

The formation of Ag₃Cit and AgC₄H₆O₂ was confirmed by FTIR spectroscopy.

Procedures for reduction of Ag⁺ ions to AgNPs in aqueous solution

Synthesis of AgNPs in the presence of β-CD and sodium hydroxide. The reduction of Ag⁺ ions was fulfilled in the presence of β-cyclodextrin with NaOH in aqueous solution as follows: 0.4 ml of β-cyclodextrin solution (0.7 µmol) with a concentration of 2 g/L, 0.2 ml of AgNO₃ (6.31 µmol), 28.9 ml of distilled water were placed to a glass tube and stirred for 2 min at room temperature and then 0.5 ml of NaOH (0.01 mmol) was added. The molar ratio of β-CD to AgNO₃ and NaOH was 1:9:15. This solution was heated at 85 °C for 6 h. The final pH of the suspensions was 5.6–5.7. This type of nanomaterial was named as . Similarly, the reduction was carried out using silver citrate and methacrylate.

For comparison, a colloid solution without β-cyclodextrin was prepared, heated and analyzed under the same conditions ( sample).

Synthesis of AgNPs in the presence of sodium borohydride. 0.4 ml of β-cyclodextrin solution (0.7 µmol) with a concentration of 2 g/L, 0.2 ml of AgNO₃ (6.31 µmol), 28.9 ml of distilled water were placed to a glass tube and stirred for 2 min at room temperature and then 0.24 ml of NaBH₄ aqueous solution (1 g/L) (6.34 µmol) was added. The molar ratio of β-CD to AgNO₃ and NaBH₄ was 1:9:9. The resulting solution was heated at 80 °С, for 1 h. This type of nanomaterial was named as .

For comparison, a solution without β-cyclodextrin was also prepared, heated and analyzed under the same conditions ( sample).

Synthesis of AgNPs in the presence of β-CD polymer. To 0.2 g of β-CD-bead polymer suspended in 100 ml of distilled water, the required amount of silver citrate (0.0032 g, 0.0167–0.079 g) was added and, after 2 min of stirring on a magnetic stirrer, the required volume of 0.1 N NaOH (0.28 mL, 1.46 mL, 6.5 mL) was added slowly. The suspension was stirred at 80 °C for 6 h. The final pH of the suspensions were 7.0-7.5. The solid phase was centrifuged and then dried at 70 °C.

Synthesis of AgNPs in the presence of pectin. 0.019 g of pectin was dissolved in 200 ml of distilled water, 0.0095 g of silver nitrate or silver citrate was added, and then 1.32 ml of NaOH (0.1 M) was added slowly while stirring on a magnetic stirrer. The resulting solution was stirred at 85 °C for 2 h. The pH of the solution after the reduction reaction was 6.90. The amount of NaOH required for the reduction of silver ions, as shown by previous experiments, is the amount needed for the saponification of pectin methoxyl groups, forming sodium pectinate and neutralizing nitric or citric acid. The AgNPs concentration was 30 ppm. Similarly, solutions with concentrations of 90 and 1000 ppm were obtained.

The synthesis of Pectin/Ag composites with a silver content of 5–20 wt% was carried out using the same technique. For this purpose, the following quantities of reagents were taken: 0.1 g silver citrate, pectin – 0.198 g, 0.495–1.056 g; 0.1 M NaOH – 13.8 ml, 25.6 ml or 48.1 ml. The pH of the solution after the reduction was 6.98, 7.91 and 8.92, respectively. The solution was evaporated in an oven at 70 °C to obtain the powder of Pectin/Ag (20 wt%) Pectin/Ag (10 wt%), and Pectin/Ag (5 wt%).

Procedures for synthesis of TiO₂/Ag nanocomposites

Synthesis of TiO₂/Ag with various Ag-containing precursors. The synthesis of TiO₂/Ag nanocomposites was achieved by utilising various silver salts (AgNO₃, Ag₃Cit and AgC₄H₆O₂). The correspondingly obtained composites were called as TiO₂/Ag_{Nit/β−CD/NaOH}, TiO₂/Ag_{Cit/β−CD/NaOH}, TiO₂/Ag_{Meth/β−CD/NaOH}.

The reduction of Ag⁺ ions in the presence of β-CD was carried out as follows: 0.5 g of TiO₂ was dispersed in 50 mL of distilled H₂O, 1.47 mL of AgNO₃ solution (C = 5.4 mg/ml), and then 2.95 ml of β-CD solution (C = 2 mg/mL) was added. After that, 0.93 ml of NaOH solution (0.05 M) was slowly added and stirred at a temperature of 85 °C for 6 h. The final pH of the suspensions was 6.5–6.8. The silver-containing nanocomposite was separated by centrifugation and dried at 70 °C (TiO₂/Ag_{Nit/CD/NaOH−1} sample). An alternative method for isolating the nanocomposite is precipitation using KCl (0.2 g), which was added to the TiO₂/Ag suspension, resulting in precipitation (TiO₂/Ag_{Nit/β−CD/NaOH−2} sample). The precipitate was then washed with a water-alcohol solution of H₂O : EtOH (25:75 v/v) and then dried at 70 °C.

The TiO₂/Ag samples using Ag₃Cit and AgC₄H₆O₂ were obtained in the same way. In this case, the all silver-containing TiO₂ nanocomposites were separated by centrifugation and dried at 70 °C.

Synthesis of TiO₂/Ag with pectin. TiO₂/Ag samples were also obtained using pectin as a reducing agent and silver nitrate or citrate. The synthesis procedure was similar to the methods described above. The sample were called as TiO₂/Ag_{Nit/Pect/NaOH}, TiO₂/Ag_{Cit/Pect/NaOH}.

Synthesis of TiO₂/Ag by UV-irradiation. 0.5 g of TiO₂ was dispersed in 50 mL of distilled H₂O, then 1.47 mL of AgNO₃ solution (C = 5.4 mg/ml) was added. The mixture was stirred with a magnetic stirrer for 5 min, and then irradiated with UV light (3 lamps, 8 W, 365 nm) for 30 min. Under irradiation, the suspension’s color changed to dark gray. The Ag-containing product was separated by centrifugation, washed with water, and dried at 70 °C. The sample was labeled as TiO₂/Ag_Nit/UV.

Synthesis of TiO₂/Ag using commercial AgNPs. The TiO₂/Ag sample a mechanical mixture of TiO₂ and commercial AgNPs (Aldrich), was prepared by mixing the components in an agate mortar with a water-ethanol suspension (50 : 50 v/v), followed by drying at 70 °C. This sample was designated as TiO₂/Ag_Aldrich.

Photocatalysis experiments

The photocatalytic properties of the prepared materials were evaluated in the reaction photodegradation using methyl orange aqueous solution. The photodestruction experiments were carried out in aqueous phase in 100 mL quartz reactor containing 40 mg photocatalysts suspended in 80 mL of the tested solution. The tested suspension was exposed to UV irradiation under magnetic stirring. The irradiation source was a system of three 8 W UV lamps with wavelengths of 365 nm (two lamps) and 254 nm (one lamp), which were placed vertically on three sides of the test reactor.

The following reagents concentrations were used in the study: TiO₂ 0.5, 1 g/L, МО – 15, 25, and 30 mg/L, distilled (pH 5.8), artesian (pH 2.5, 4.8, 5.8, 7.5), and river (pH 7.00) water samples.

To study the kinetics of photodegradation the MO concentration was measured at certain intervals (after centrifugation of the solid phase) and determined spectrophotometrically in 464–504 nm.

Environmental Water Sample Assay. The real environmental samples (Table 1) were collected from Kyiv (Ukraine), including artesian and urban river water samples. These samples were taken from an artesian well (50 m deep) in Akademcity (Kyiv district area) and from the Dnipro River (in the centre of the city).

Table 1.

Physicochemical characteristics and chemical composition of the water sources.

Parameter	Unit	River Water	Artesian Water
pH	-	7.8 ± 0.2	7.2 ± 0.1
Conductivity	µS/cm	450 ± 15	820 ± 25
Total Organic Carbon (TOC)	mg/L	5.4 ± 0.4	0.8 ± 0.1
Total Hardness	mg-eq/L	4.2	6.5
Anions
Chloride (Cl⁻)	mg/L	25.4	12.5
Sulfate (SO₄²⁻)	mg/L	48.2	110.3
Bicarbonate (HCO₃⁻)	mg/L	185.0	320.0
Nitrate (NO₃⁻)	mg/L	12.1	2.4
Cations
Calcium (Ca²⁺)	mg/L	65.2	95.8
Magnesium (Mg²⁺)	mg/L	18.5	24.3
Sodium (Na⁺)	mg/L	15.2	45.1
Potassium (K⁺)	mg/L	3.4	1.2

The following reagents concentrations were used in the study: TiO₂ 0.5, 1 g/L, МО – 15, 25, and 30 mg/L artesian (pH 2.5, 4.8, 5.8, 7.5), and river (pH 7.00) water samples.

The destruction efficiency (D, %) of photocatalysis experiments for the obtained samples was defined using the following equation:

where and are the concentrations of MO at initial and any time contact with the tested photocatalyst, respectively.

Results and discussion

Before carrying out the Ag⁺ reduction procedure on the surface of TiO₂, the characteristics of the model reaction where Ag⁺ ions are reduced without the addition of TiO₂ were investigated.

Synthesis and characterization of AgNPs in solution

AgNPs were prepared by the chemical reduction of various silver (I) precursors (AgNO₃, Ag₃Citr and AgMeth) and different reduction agents including NaBH₄, NaOH, and UV-radiation. The effect of various synthetic parameters such as temperature, time, initial concentration of silver (I) sources, and the addition of β-CD and its derivatives was also assessed.

The color of the solutions changed from colourless to yellow and then to light brown as a result of the reduction of Ag⁺ ions by reduction agents, including NaBH₄, NaOH and UV-radiation. The pH of the resulting solution was 5.6–5.7.

The AgNPs formation was monitored by changing the intensity and position of the surface plasmon resonance (SPR) band in the UV-Vis spectra of colloids (Fig. 1).

Fig. 1.

UV-Vis spectra of AgNPs obtained by reduction of Ag⁺ ions with NaOH and β-CD/NaOH at various time and temperatures 85 °C (a) and 90 °C (b).

The spectra of silver-based colloids contain a strong SPR band at around 420 nm, confirming that Ag⁺ ions were reduced to Ag⁰ in the aqueous phase (Fig. 1). Most of the Ag⁺ ions were reduced for 60 min at 85 °C, but for complete reduction, the reaction was performed up to 360 min (Fig. 1a). Comparison of the UV spectra of the solution obtained with NaOH alone indicates a significant acceleration of the reduction of Ag⁺ ions in the presence of cyclodextrin molecules, which may be related to the stabilization of the obtained nanoparticles by hydroxyl groups of β-CD. Similarly, UV-Vis spectra of AgNPs obtained at 90 °C show SP with SPR bands at 400–425 nm (Fig. 1b). The difference between the two solutions in this case was not so noticeable, but the presence of β-CD also accelerated the reduction of Ag⁺ ions, the SPR peak was slightly narrower. Therefore, analysis of the UV-Vis spectra of the AgNPs indicated that the optimal temperature for the reaction was 85 °C, carried out over a period of 60 min. These conditions were used in subsequent syntheses of AgNPs.

The effect of the initial concentration of the AgNO₃ solution on the formation of AgNPs with β-CD/NaOH was also studied (Fig. 2). The maximum absorption bands shifted from 411 nm to 424 nm as the AgNO₃ concentration increased from 0.02 mM to 0.25 mM. This indicates an increase in nanoparticle size, while at C < 0.14 mM, practically no reduction takes place. From the UV-Vis spectra of the colloids, it was found that the optimal concentration of AgNO₃ for the synthesis of AgNPs was 0.2 mM.

Fig. 2.

UV-Vis spectra of Ag_{Nitr/β-CD/NaOH} at different concentrations of AgNO₃.

In case of AgNPs synthesized with pectin (Fig. 3), the SPR bands in the UV-Vis spectra remained consistently between 396 and 400 nm as the AgNO₃ concentration increased from 0.02 mM to 0.25 mM. This consistent SPR band position suggests the formation of spherical nanoparticles of relatively small size (5–8 nm)^35,5140,56. These data were confirmed by TEM method (Fig. S1). The same effect was observed for AgNPs obtained by reduction of Ag₃Cit solutions (30–90 ppm) with Pectin/NaOH.

Fig. 3.

UV-Vis spectra of AgNPs obtained using different concentrations of AgNO₃ and Ag₃Cit solutions in the presence of Pectin/NaOH (Spectra recorded at C_AgNPs = 30 ppm).

The effect of β-cyclodextrin on the reduction of Ag⁺ ions using NaBH₄ is shown by UV-Vis spectra (Fig. 4). In this case, the SPR band is observed at 398 nm and is narrower, which is due to the formation of smaller nanoparticles.

Fig. 4.

UV-Vis spectra of obtained and samples.

The AgNPs obtained by the addition of various Ag(I) precursors (AgNO₃, Ag₃Cit or AgMeth) dropwise into an aqueous solution of sodium borohydride, NaOH, β-CD or their mixtures at 85 °C were also studied by DLS analysis (Fig. 5). The particle size distributions by number of AgNPs obtained using AgNO₃ solution with β-CD/NaOH, neat NaOH, β-CD/NaBH₄, and neat NaBH₄ were found between 2.5 and 8.7 nm. The average particle size, estimated by their number and hydrodynamic diameter, was 2.5 nm, 8.7 nm, 4.2 nm, 4.3 nm and 83.2 nm, 4912.9 nm, 39.6, and 34.1 for , , Ag_/NaOH, and , respectively. The DLS spectra of several colloids show several peaks that correlate to the amount of components in the reaction mixtures.

Fig. 5.

Particle size distribution by number (a) and intensity (b) of AgNPs obtained with AgNO₃ and β-CD/NaOH, NaOH, β-CD/NaBH₄, NaBH₄.

Silver(I) ions originated from AgNO₃ and Ag₃Cit were also reduced involving some other β-CD derivatives, in particular carboxymethyl (CM)-, methyl (Me)-, hydroxypropyl cyclodextrin (HP-CD) (Fig. 6). Based on the intensity and position of the SPR band, the most effective is the application of CM-β-CD, apparently due to the more efficient stabilization of AgNPs by its carboxyl groups, while Me-β-CD, in which the hydroxyl groups are replaced by methyl groups, is the least effective.

Fig. 6.

UV-Vis spectra of AgNPs obtained by reduction of Ag⁺ ions with β-CD and its derivatives after 6 h of reduction at 85 °C.

The particle size distribution is shown in Fig. 7. The smallest nanoparticle sizes were prepared using carboxymethyl and maleoyl-β-cyclodextrin − 4.6 nm and 2.2 nm, respectively.

Fig. 7.

Particle size distribution by intensity (a, c) and number (b, d) of AgNPs obtained with CM-CD, HP-CD, Me-CD, SB-CD and NaOH.

Thus, the AgNPs obtained by reducing of Ag⁺ ions with NaOH or NaBH₄ in β-cyclodextrin aqueous solution were taken for further investigations.

The pectin and β-CD bead polymer (“Cyclolab”, β-CD cross-linked with epichlorohydrin) composites with different contents of AgNPs were characterized by powder XRD analysis in the wide range region (Fig. 8).

Fig. 8.

XRD patterns of Ag_{Cit/NaOH/CD bead pol} (a) and Ag_{Cit/NaOH/Pectin} (b) composites with various concentration of AgNPs.

As shown in Fig. 8a, XRD data indicates that an increase in silver concentration results in broader and more diffuse peaks corresponding to the crystalline structure of pectin, likely due to a decrease in the size of the pectin crystallites. At the same time, the width of the peaks corresponding to silver decreases as its content increases, which is obviously due to an increase in the size of the areas of coherent scattering of silver particles. Analysis of the physical width of the diffraction peaks from the silver phase using the Scherrer formula revealed that the average size of silver crystallites gradually increases from 7.8 to 9.3 nm as the silver content increases from 5% to 20% (Table 2).

Table 2.

The DLS data of the prepared AgNPs.

Sample	Mean particle size, nm	Hydrodynamic diameter, nm	Particle size range, nm	ξ-potential, mV	Polydispersity index
AgNit/β−CD/NaOH	2.5	83.2	2.19–3.29 16.6-360.8	−23.7	27.3
AgMeth/CDMal/NaOH	17.9	82.5	14.1-1121.4 1823.2-6144.2	−22.9	19.9
AgCit/β−CD/NaOH	7.2	70.3	5.8-204.7	−16.2	26.7
AgNit/CDMal/NaOH	2.2	72.2	1.9–3.3 14.1-391.3	−25.3	26.6
AgCit/CDMal/NaOH	7.3	77.3	5.8–273.0	−25.4	28.2
AgNit/Pect/NaOH	9.5	92.8	7.4–261.0	−30.9	27.1
AgCit/Pect/NaOH	4.9	70.3	3.9–261.0	−18.8	26.3

Similar trends are seen for CD bead pol/AgNPs composites (Fig. 8b; Table 3), where the average size of silver crystallites increases as the concentration rises from 1 to 25 wt%. Furthermore, this increase is accompanied by a reduction in the volume fraction of the amorphous phase of cyclodextrin, resulting in the crystallinity of the CD-Ag composite rising from 8 to 95%. For both polymer matrices, the size of silver nanoparticle crystallites was significantly smaller than for pure NPs synthesized in the presence of the neat β-CD. This indicates the matrix’s influence on crystallite formation.

Table 3.

Crystallite sizes of AgNPs on pectin and CD-polymer matrix according XRD data.

Sample	AgCit/β−CD/NaOH	CD bead pol/AgNPs			Pec/AgNPs
		1 wt%	10 wt%	25 wt%	5 wt%	10 wt%	20 wt%
D, nm	19.2	9.1	10.9	12.5	7.8	8.7	9.3

FTIR spectra of Pectin-Na, Pectin-Ag, and CD-bead polymer-Ag nanocomposites are shown in Fig. 9. The bands at 3439–3416 cm^− 1 and 2932–2927 cm^− 1 are assigned to the stretching vibrations of OH groups and C–H bonds, respectively. The strong bands at 1641–1605 cm^− 1 and 1410–1411 cm^− 1 are associated with the stretching vibrations of carboxylate groups ν_as(COO⁻) and ν_s(COO⁻). These groups form during the saponification of pectin ester groups under alkaline conditions. The bands in the range of 1200–1000 cm^− 1 correspond to the C–O stretching vibrations of the glycoside ring (1017 cm^–1, 1036 cm^–1) and the glycoside bridge (1141 cm^–1, 1164 cm^–1). The absence of the band at 1744 cm⁻¹ in the Pectin-Na and Pectin-Ag spectra indicated that all the ester groups have been saponified to form COONa groups. The shift of ν_as and ν_s in the spectra of silver-containing samples indicates the interaction between these functional pectin groups and AgNPs. The presence of such bands in the spectra of CD-bead pol/AgNPs indicates the oxidation of the OH groups in the β-CD during the synthesis of AgNPs.

Fig. 9.

FTIR spectra of Pectin/Ag (a) and CD bead pol/Ag (b) composites.

Thus, the established relationships for the AgNPs and the methods for their preparation were utilized and further refined during the synthesis of TiO₂/Ag composites.

Preparation and characterization of various TiO₂/Ag composites

The TiO₂/Ag composites were prepared by reducing various sources of Ag⁺ ions with NaOH or NaBH₄ in β-cyclodextrin aqueous solution in the presence of TiO₂ at 85 °C. To compare, TiO₂/Ag_Nit/UV TiO₂/Ag_Aldrich samples were also studied.

To determine the structural characteristics of the obtained TiO₂/Ag composites, XRD analysis was performed (Fig. 10).

Fig. 10.

Powder XRD patterns of starting AgNPs and TiO₂ samples compared to resulted TiO₂/Ag nanocomposites and corresponding standards (Ag, Ag₂O and AgCl) at 2θ from 24 to 28° (a) and from 26 to 50° (b).

XRD profile of pristine TiO₂ presents several sharp peaks at 2θ = 25.26, 37.80, 48.00 (Fig. 10), which are assigned to the Miller indices (101), (004), (200) of the anatase phase as reported in ICSD no. 9854. Anatase is known to be the most photocatalytic active polymorph of TiO₂¹². A very small amount of rutile phase is also seen in TiO₂(ICSD no. 9161), as represented by small diffraction peaks at 27.4, 36.0, and 41.2°, assigned to (110), (101), and (111) planes, respectively. The most TiO₂/Ag nanocomposites synthesized via different routes exhibited low-intensity peaks 38.41°and 44.20°, which corresponds to the (111) and (200) planes of silver face-centered cubic lattice phase (Silver 3 C, ICSD no. 64706) (Fig. 10b). Nonetheless, the characteristic peaks of the AgNPs appeared less prominent, potentially due to the low concentration of nanoparticles on the TiO₂ support or their lattice incorporation^77,78. Other contributing factors may include the small crystallite size (5–10 nm), which diminishes long-range crystalline order and causes spectral overlap with TiO₂ reflections. The XRD data also demonstrate that Ag₂O did not form in the obtained TiO₂/Ag nanocomposites.

For and samples, three minor diffraction peaks were observed at 2θ values of 27.8°, 32.2°, and 46.2° (Fig. 10b), corresponding to the crystalline phase of AgCl (ICSD no. 56538). The presence of AgCl was identified as a result of the precipitation of samples with KCl during the synthesis procedure, indicating that this specific synthesis route leads to formation of additional impurities.

Compared to pristine TiO₂, the characteristic (101) reflection of the anatase phase in the Ag-containing nanocomposites exhibits a discernible shift toward higher diffraction angles (2θ) by 0.05–0.13° (Fig. 10a). This means that Ag doping could reduce the d-spacing of TiO₂ that calculated according to the Bragg equation (Table 4). A possible explanation for this could be the formation of oxygen vacancies due to the doping by silver nanoparticles^79,80. Based on the fact that Ag⁺ has a much larger ionic radius (1.26 Å) than Ti⁴⁺ (0.68 Å), the observed slight decrease in d-distance and lattice parameters (mainly c) also probably indicates that AgNPs can be incorporated into the crystal lattice, occupying an interstitial position, which can cause internal stresses leading to lattice compression. Instead, substitution of titanium atoms would increase these parameters and expand the crystal lattice^77,81. In the FTIR spectrum of pristine TiO₂ (Fig. S4), two bands in the low-frequency region at 520 cm^–1 and 682 cm^–1 were observed, which are assigned to Ti–O–Ti bridging stretching and Ti–O stretching mode, respectively⁸². In the spectra of silver-containing composites, the band at 520 cm^–1 shifted toward higher frequencies (540–580 cm^–1), while the Ti–O–Ti valence band at 682 cm^–1 did not change position. According to the FTIR and XRD data, the AgNPs are located in the interstitial space on the TiO₂ surface. The shift towards the high-frequency region indicates a strengthening of the Ti–O–Ti bonds. This phenomenon is most pronounced in the nanocomposites synthesized with β-СD and pectin, suggesting a significant structural reorganization. Such results likely stem from modifications to the anatase crystal lattice triggered by its interaction with AgNPs. The incorporation of AgNPs appears to induce local deformations within the TiO₂ matrix, altering the Ti–O–Ti bond angles, thereby strengthening the bonds and increasing the corresponding vibrational frequencies⁸⁰.

Table 4.

Structural parameters of the as-prepared TiO₂-based samples according XRD data.

Sample	2θ, °	FWHM, °	D, nm	d, Å	Lattice parameters
					a, Å	c, Å	V, Å3
TiO 2	25.26	0.2891	28.16	3.521	3.783	9.627	137.77
	25.34	0.1622	50.21	3.510	3.779	9.476	135.32
	25.34	0.1576	51.67	3.513	3.781	9.510	135.95
	25.33	0.1649	49.38	3.512	3.780	9.490	135.60
	25.31	0.1622	50.20	3.514	3.780	9.502	135.77
	25.33	0.1571	51.83	3.512	3.781	9.522	136.13

FWHM – full width at half maximum of peak; D – crystallite size obtained by Scherer; d – interplanar spacing; V – cell volume.

The narrowing of the XRD diffraction patterns for all Ag-decorated TiO₂ samples indicates a significant increase in the average crystallite size (Table 4). The average crystallite size of nanomaterials was calculated using the Scherrer equation based on the most intense diffraction peak at 2θ = 25.3°, corresponding to the (101) plane of the anatase phase. The D value increased from 28.16 nm for pristine TiO₂ to approximately 50±2 nm for the TiO₂/Ag composites. The sample synthesized via UV-assisted reduction exhibited a lowest crystallite size of 49.38 nm. At the same time, the utilization of β-CD as a reducing agent yielded the largest crystallite size of 51.83 nm. Thus, the presence of AgNPs promotes the crystal growth of the TiO₂-based nanocomposites. The increase in its crystallinity and crystallite size could results in lowering their combination rate, hence provide better photocatalytic activity.

Additionally, the morphology and size of the all TiO₂-based samples was investigated using scanning electron microscopy (Fig. S2 and Fig. S3). The initial TiO₂ particles were found to be predominantly spherical, with an average diameter of approximately 50 nm (Fig. S2). SEM images of the TiO₂/Ag nanocomposites reveal a different morphology with highly dispersed particles (Fig. S3). The primary TiO₂/Ag particles are found in near-spherical form. However, they exhibit a strong tendency to form dense agglomerates with quasi-spherical form. The surfaces of these agglomerates are rough and heterogeneous, indicating the presence of fine AgNPs immobilized on the TiO₂ support. The particle sizes were found to range from 40 nm to 280 nm for the obtained nanocomposites. In this case, the mean particles of the samples were decreased in order as follows: (142 nm) > (140 nm) > (129 nm) > TiO₂/Ag_{Nit/NaOH//β−CD} (128 nm) > (127 nm) > (125 nm) > TiO₂ (50 nm). Thus, the mean particle size for all synthesized nanocomposites (125–142 nm) was substantially larger than that of pristine TiO₂ (50 nm). This order can be attributed to the successful deposition of AgNPs species on the TiO₂ surface and the formation of decorated layer. The highest aggregation was observed in samples synthesized with NaOH without β-CD ( sample). The rapid kinetics of chemical reduction by NaOH often leads to high local supersaturation, favoring the formation of large aggregates. Samples prepared with pectin exhibited the smallest average particle sizes (125–127 nm) and, crucially, a lower degree of aggregation according to SEM analysis. This is likely due to the high density of functional groups (carboxyl and hydroxyl) in pectin, which act as a robust steric and electrostatic barrier, preventing the aggregation of AgNPs. The presence of pectin effectively mitigates the high surface energy of the TiO₂/Ag samples, leading to more uniform nanocomposites with the greatest amount of AgNPs (0.95–0.96 at%) confirmed by EDX analysis. This observation results in the provision of both a high specific surface area and favorable interfacial contact between TiO₂ matrix and AgNPs, thus enhancing the material’s photocatalytic properties.

The DLS method was used for a qualitative evaluation of particle size of TiO₂ and TiO₂/Ag samples in the aqueous solution (Fig. 11).

Fig. 11.

Particle size distribution by intensity (a) and number (b) of obtained TiO₂/Ag composites compared to initial TiO₂.

The particle size ranges from 100 to 1000 nm. For pure TiO₂ a distribution with maxima at 51.8 and 429.9 nm and a mean hydrodynamic diameter of 379.3 nm was found. For the TiO₂/Ag composites the maxima are generally close to the peak of pure TiO₂. The maximum in the distribution curve for TiO₂/Ag from nitrate (426.5 nm) is slightly lower compared to pure TiO₂, although the average particle size and hydrodynamic diameter are still larger of the composite. TiO₂/Ag from silver methacrylate and citrate have slightly larger particles than pure TiO₂ and TiO₂/Ag_/Nit/CD. However, these samples have a narrower distribution, which may indicate that the silver nanoparticles contribute to the formation of more homogeneous particles. The particle size distribution for the TiO₂Ag_Cit/Pect sample was even narrower, with an average particle size of 377.3 nm. The sample obtained from Ag₃Cit shows a slightly narrower distribution with the lowest polydispersity index. It should also be noted that the nanocomposite obtained from silver methacrylate has the highest negative zeta potential as well as samples obtained with pectin as reducing agent, indicating greater stability of such composites (Table 5).

Table 5.

The DLS data of the prepared TiO₂-based samples.

Sample	Mean particle size, nm	Hydrodynamic diameter, nm	Particle size range, nm	ξ, mV	PDI
TiO 2	55.4/421.4	379.3	34–953	−32.0	0.23
	485.9	445.9	160–1216	−32.3	0.21
	486.6	489.0	205–1034	−34.7	0.25
	501.8	501.5	222–954	−31.7	0.18
	377.3	403.2	189–690	−40.2	0.23

PDI – polydispersity index.

The surface potential significantly affected the adsorption of organic components and its intermediates on the surface of the photocatalyst during photoreaction. The variation of ξ-potential measurements of samples is shown in Table 5. All nanocomposites exhibited a strong negative surface charge ranging from − 32.0 mV to -40.2 mV. Specifically, the pectin-stabilized sample () demonstrated superior stability (ξ = -40.2 mV) due to the synergistic effect of electrostatic repulsion and steric hindrance provided by the polysaccharide chains, effectively preventing particles sedimentation.

Photocatalytic activity of TiO₂/Ag nanocomposites toward dye degradation under UV light in the model and real water samples.

The photodegradation of MO dye under UV light was used to assess the photocatalytic activity of the obtained catalysts. For comparison, pristine TiO₂ was used as benchmarks.

Photocatalytic activities in the model solution

Kinetics of photodegradation of Methyl Orange in aqueous solution. The UV-Vis spectra of MO solutions with an initial concentration of 15 mg/L after their UV-irradiation for 10–40 min (Fig. 12). As shown, after 20 min of irradiation, the solution was completely discolored, when TiO₂/Ag_{Nit/CD/NaOH−1} photocatalyst was employed, although there was a shoulder band at 350 nm that disappeared with subsequent irradiation process. When pure TiO₂ was applied, only 39.3% of MO was decomposed after 30 min.

Fig. 12.

UV-Vis spectra of MO solutions after irradiation using pure TiO₂/Ag_{Nit/β−CD/NaOH} (a) and TiO₂(b) samples in the aqueous solution (Conditions: C₀(MO) = 15 mg/L, sample – distilled water, V(total) = 40 mL).

The UV-Vis spectra of solutions after 60 min of irradiation with the initial MO concentration equals 30 mg/L using different catalysts (Fig. 13). For comparison, the sample is also tested. In general, quite good results were obtained for the TiO₂/Ag_CD/NaOH and TiO₂/Ag_UV samples. However, for the last sample, the intensity was quite high at about 200 nm, as in general for other compounds. This implies that certain organic compounds remain in solution after the decomposition of MO.

Fig. 13.

UV-Vis spectra of MO solutions after irradiation for 60 min using various prepared samples (Conditions: C₀(MO) = 30 mg/L, medium - distilled water, V = 40 mL).

Evaluation of photocatalytic activities in the real water solution: In order to determine the effect of the β-CD macrocycle on the photocatalytic properties of silver-containing samples, the study was first carried out on samples containing only silver nanoparticles reduced from different silver salts in the presence and absence of β-cyclodextrin. A specimen containing silver nanoparticles (supplied by Aldrich) mechanically mixed with TiO₂ was also tested for comparison. It was shown that the samples originated with silver methacrylate had a higher photocatalytic activity for methyl orange (Fig. 14).

Fig. 14.

Degradation of methyl orange in the artesian water in the presence of various photocatalysts at pH 2.5 (a), 4.8 (b), 5.8 (c) and 7.5 (d) (Conditions: С₀(MO) = 25 mg/L; weight 40 mg, volume 80 mL).

The greatest influence of β-CD on the photocatalytic properties demonstrates the sample prepared with AgNO₃, which in this case is also accounted for by the incomplete reduction of silver ions (without β-CD). Instead, when silver citrate and methacrylate were used, the difference in photocatalytic properties for samples synthesized in the presence and without β-CD was not as significant as in the case of Ag nitrate. The positive effect of β-CD on the photocatalytic activity is probably due to the stabilization of silver NPs by macromolecules, as well as by citrate and methacrylate anions.

A comparison of the photodegradation course of MO in artesian and river water samples with TiO₂ and TiO₂/Ag_CD/NaOH was studied (Fig. 15). For the pure TiO₂ the photodegradation was rather slow, and for 100 min of irradiation in the system still remained 48% of the MO’s initial concentration, while applying the sample obtained with β-CD/NaOH led to complete decomposition. Also, the photodegradation in the river water was slightly slower up to 60–80 min compared to the artesian water, and based on the absorption maximum at 194 nm some aliphatic products are present in the solution.

Fig. 15.

UV-Vis spectra of MO solutions after irradiation for 60–100 min using TiO₂ and TiO₂/Ag_CD/NaOH (a-c); C₀(MO) = 30 mg/L, C(TiO₂) = 1 g/L, V = 40 ml, medium – artesian (AW) (b) and river water (RW) (c).

The kinetics of MO photodegradation in the initial artesian water (pH 7.5) and in water acidified to pH 5.8 and 4.8 are shown in Fig. 16. The change in the pH of the reaction medium significantly affects the efficiency of photocatalytic degradation of methyl orange in the presence of neat TiO₂. The increased degradation rate in an acidic medium (pH 2.5) is due to the positive charge of the TiO₂ surface, since the pH of the zero charge point is ≈ 5.6–6.8^83–85, and to the enhanced adsorption of the anionic dye. In alkaline conditions, electrostatic repulsion reduces photocatalytic activity. pH also changes the spectral and acid-base properties of MO; in an acidic environment, MO transitions to a quinoid form, which facilitates the breakdown of the chromophore system under the action of photo-generated radicals, while in alkaline conditions, the MO molecule is more stable to photodegradation. Modification of TiO₂ with Ag nanoparticles could shift the zero charge point of the photocatalyst towards slightly alkaline values (≈ 6.6–7.3)⁸⁵, thereby expanding the range of conditions under which the catalyst surface is positively charged. This enhances the adsorption of anionic methyl orange under neutral conditions and maintains an active photocatalytic reaction even at slightly alkaline pH by reducing carrier recombination and promoting radical formation.

Fig. 16.

UV-Vis spectra of MO in artesian water after irradiation for 15–120 min using TiO₂ and TiO₂/Ag_Meth/NaOH at pH 5.8 (a) and 7.5 (b) (Conditions: C₀(MO) = 25 mg/L, weight 40 mg, volume 80 mL at room temperature).

Conclusions

The effect of β-cyclodextrin and its derivatives on the processes of silver ion reduction in aqueous medium was investigated. UV-Vis spectroscopy data showed that the presence of cyclodextrin moieties significantly accelerated the reduction of silver ions compared to a solution without ones, even at a low molar ratio of CDs to Ag of 1:9 and at a neutral pH. The average size of the nanoparticles, determined from Dynamic Light Scattering method, was in the range of 2–20 nm, depending on the salt anion and CD derivative, the SPR band of AgNPs in the UV spectra was within the range of 412–420 nm. According to TEM data, the nanoparticles were 5.1 and 7.2 nm in size when pectin was used as a reducing agent at a mass ratio of 2:1, with SPR bands at 398 and 402 nm, respectively, for citrate and silver nitrate as metal precursors. The sizes of these silver nanoparticles correlate with the XRD data for pectin/AgNPs and CD-beads/AgNPs nanocomposites – 7.8–9.3 nm and 9.1–12.5 nm, depending on the silver content (1–25 wt%). Using these reducing agents (CDs and pectin) and silver precursors such as nitrate, citrate and methacrylate, TiO₂/Ag nanocomposites with a silver content of 1 wt% were synthesized. Silver-containing nanocomposites based on TiO₂ demonstrated high photocatalytic activity towards methyl orange dye. Better photodegradation of MO was observed in samples prepared from silver methacrylate. In this case, complete decolorization of the MO solution occurred within 30 min at pH 2.5 and 70 min at pH 7.5, which is 2.5 times and 3 times faster than with pure TiO₂.

Supplementary Information

Below is the link to the electronic supplementary material.

Author contributions

***Serhii Kobylinskyi*** - investigation, writing – original draft, conceptualization, formal analysis; ***Larisa Kobrina*** - investigation, visualization; ***Sergiy Polishchuk*** - investigation, software; ***Natalia Kobylinska*** – investigation, formal analysis; ***Anton Tymoshyk*** *-* investigation, visualization; ***Sergii Riabov*** - conceptualization, editing text, project administration, supervision.

Data availability

Data are provided within the manuscript.

Declarations

Competing interests

The authors declare no competing interests.