Formate incorporation in the structure of Ca-deficient apatite: Rietveld structure refinement

Efficacy of sintered Zinc-doped fluorapatite scaffold as an antimicrobial regenerative bone filler for dental applications

OBJECTIVES

The objective of this study was to assess whether zinc-doped fluorapatite (ZnFA) could serve as an effective antimicrobial dental bone filler for bone regeneration compared to autografts.

METHODS

FA and 2 % zinc-doped FA (2ZnFA) were synthesized and characterized in-house. Compressed and sintered FA and 2ZnFA disks were incubated with bacteria to assess antimicrobial properties. Adipose-derived stem cells were cultured on these discs to evaluate the surfaces' ability to support cell growth and promote osteogenic differentiation. Surfaces exhibiting the highest expressions of the bone markers osteopontin and osteocalcin were selected for an in vivo study in a rat mandibular defect model. Twenty rats were divided into 5 groups, equally, and a 5 mm surgical defect of the jaw was left untreated or filled with 2ZnFA, FA, autograft, or demineralized bone matrix (DBM). At 12 weeks, the defects and surrounding tissues were harvested and subjected to microCT and histological evaluations.

RESULTS

Standard techniques such as FTIR, ICP-MS, fluoride probe, and XRD revealed the sintered FA and ZnFA's chemical compositions and structures. Bacterial studies revealed no significant differences in surface bacterial adhesion properties between FA and 2ZnFA, but significantly fewer bacterial loads than control titanium discs (p < 0.05). Cell culture data confirmed that both surfaces could support cell growth and promote the osteogenic differentiation of stem cells. MicroCT analysis confirmed statistical similarities in bone regeneration within FA, 2ZnFA, and autograft groups.

CONCLUSION

The data suggests that both FA and 2ZnFA could serve as alternatives to autograft materials, which are the current gold standard. Moreover, these bone fillers outperformed DBM, an allograft material commonly used as a dental bone void filler.

CLINICAL SIGNIFICANCE

The use of FA or 2ZnFA for treating mandibular defects led to bone regeneration statistically similar to autograft repair and significantly outperformed the widely used dental bone filler, DBM. Additional translational research may confirm FA-based materials as superior substitutes for existing synthetic bone fillers, ultimately enhancing patient outcomes.

A novel route to enhance the dissolution of apatite: Structural incorporation of hydrogen phosphate

Potential use of hydroxyapatite nanoparticles (HANPs) [Ca₁₀(PO₄)₆(OH)₂] as slow P-release fertilizer (SRF) has recently attracted wider attention. However, commercially available HANP (with Ca/P ratio = 1.667) is the least soluble calcium phosphate and thus limits its full potential as an SRF in agronomic applications. In this research, we sought to enhance the dissolution rate of HANPs by enriching hydrogen phosphate (HPO₄^2-) species in the phosphate (PO₄^3-) structural sites. Seven different types of pure crystalline HANPs were synthesized at a range of Ca/P ratio from 1.46 (at pH 6.0) to 2.10 (at pH 12.0). Complementary results from FTIR and solid-state ³¹P MAS NMR spectroscopies showed that HPO₄^2- species is most abundant in HANPs crystallized at pH 6.0 and gradually depleted at higher pH products. The rate of depletion of HPO₄^2- species is proportional to the increase in carbonate incorporation into the HANP lattice, which preferentially forms B-type carbonated HANPs. The enhanced dissolution rate of HANPs due to hydrogen phosphate incorporation was tested using a flow-through macro-dialysis system that limits the partial transition of HANPs to other solid phases, which otherwise interfere with dissolution. The results show that the dissolution rate of HANPs increased with decreasing pH of synthesis and was highest in HANPs at pH 6.0. The dissolution rate differed by ten times between HANPs synthesized at pH 7.0 and 10.0. Overall, the atom-efficient synthetic route developed and the ability to tune the dissolution rate of HANPs are significant steps forward in improving the P-release efficiency of a potent SRF and is expected to contribute to efforts toward enhancing agricultural sustainability.

Fabrication and Characterization of a Nanofast Cement for Dental Restorations

This study was aimed at fabricating and evaluating the physical and bioproperties of nanofast cement (NFC) as a replacement of the MTA. The cement particles were decreased in nanoscale, and zirconium oxide was used as a radiopacifier. The setting time and radiopacity were investigated according to ISO recommendations. Analysis of color, bioactivity, and cytotoxicity was performed using spectroscopy, simulated body fluid (SBF), and MTT assay. The setting time of cement pastes significantly dropped from 65 to 15 min when the particle sizes decreased from 2723 nm to 322 nm. Nanoparticles provide large surface areas and nucleation sites and thereby a higher hydration rate, so they reduced the setting time. Based on the resulting spectroscopy, the specimens did not exhibit clinically noticeable discoloration. Resistance to discoloration may be due to the resistance of zirconium oxide to decomposition. Scanning electron microscopy (SEM), X-ray diffraction (XRD), and infrared spectroscopy (FTIR) examinations of the immersed SBF samples showed apatite formation that was a reason for its suitable bioactivity. The results of cell culture revealed that NFC is nontoxic. This study showed that NFC was more beneficial than MTA in dental restorations.

Irreversible adsorption of acidic, basic, and water gas molecules on calcium-deficient hydroxyapatite

Hydroxyapatite [Ca10(PO4)6(OH)2, HAP] has P-OH Brønsted acidic sites, Ca2+ Lewis acidic sites, and OH- and O2- basic sites on which acidic and basic gas molecules can be selectively adsorbed, and has no micropore onto which various molecules adsorb regardless of the chemical properties of gas molecules. The interaction between the surface sites and acidic and basic gas and water molecules has been investigated by evaluating the adsorption properties of various molecules on the surfaces of calcium-deficient HAP. The specific adsorption sites were assessed by examining the reversible and irreversible adsorption of NH3, CO2, aldehydes, and water vapor on HAP at the temperature of 298 K, using two HAP samples with different Ca/P ratios, but similar structures and surface areas: Ca-deficient HAP with an extreme lower Ca/P ratio (named P-HAP) and one with a higher Ca/P ratio (named C-HAP). Irreversible adsorption of NH3 on C-HAP is attributed to the adsorption on both Ca2+ Lewis acidic and P-OH Brønsted acidic sites. Irreversible adsorption on P-HAP is attributed to the adsorption on P-OH Brønsted acidic sites only. Irreversible adsorption of CO2 occurred on C-HAP only, and preferentially on OH- basic sites. Acetaldehyde undergoes a catalytic reaction over both OH- basic sites and surface P-OH Brønsted acidic sites at 298 K. Water irreversible adsorption was extensively observed for P-HAP, and water was barely desorbed at low pressures. In situ powder X-ray diffraction showed an asymmetric expansion of the lattice in the [100] direction, indicating that water was incorporated into P-HAP crystals, especially on structural OH- sites. Irreversible adsorption of acidic and basic molecules was therefore less observed on P-HAP than on C-HAP, but P-HAP had considerable irreversible adsorption of water vapor with associated asymmetric lattice expansion. The incorporation of water vapor was first observed and could be useful to improve adsorption or catalytic performance with the mediation of water vapor and/or hydration.

Amorphous Calcium Carbonate Based-Microparticles for Peptide Pulmonary Delivery

Amorphous calcium carbonate (ACC) is known to interact with proteins, for example, in biogenic ACC, to form stable amorphous phases. The control of amorphous/crystalline and inorganic/organic ratios in inhalable calcium carbonate microparticles may enable particle properties to be adapted to suit the requirements of dry powders for pulmonary delivery by oral inhalation. For example, an amorphous phase can immobilize and stabilize polypeptides in their native structure and amorphous and crystalline phases have different mechanical properties. Therefore, inhalable composite microparticles made of inorganic (i.e., calcium carbonate and calcium formate) and organic (i.e., hyaluronan (HA)) amorphous and crystalline phases were investigated for peptide and protein pulmonary aerosol delivery. The crystalline/amorphous ratio and polymorphic form of the inorganic component was altered by changing the microparticle drying rate and by changing the ammonium carbonate and HA initial concentration. The bioactivity of the model peptide, salmon calcitonin (sCT), coprocessed with alpha-1-antitrypsin (AAT), a model protein with peptidase inhibitor activity, was maintained during processing and the microparticles had excellent aerodynamic properties, making them suitable for pulmonary aerosol delivery. The bioavailability of sCT after aerosol delivery as sCT and AAT-loaded composite microparticles to rats was 4-times higher than that of sCT solution.

Control of calcium accessibility over hydroxyapatite by post-precipitation steps: influence on the catalytic reactivity toward alcohols

Hydroxyapatites exhibit Ca²⁺ and POH surface acid sites, which relative accessibilities can be varied by synthesis to tune the catalytic reactivity toward alcohols.

Structural water in carbonated hydroxylapatite and fluorapatite: confirmation by solid state (2)H NMR

Water is well recognized as an important component in bone, typically regarded as a constituent of collagen, a pore-filling fluid in bone, and an adsorbed species on the surface of bone crystallites. The possible siting and role of water within the structure of the apatite crystallites have not been fully explored. In our experiments, carbonated hydroxyl- and fluorapatites were prepared in D(2)O and characterized by elemental analysis, thermal gravimetric analysis, powder X-ray diffraction, and infrared and Raman spectroscopy. Two hydroxylapatites and two fluorapatites, with widely different amounts of carbonate were analyzed by solid state (2)H NMR spectroscopy using the quadrupole echo pulse sequence, and each spectrum showed one single line as well as a low-intensity powder pattern. The relaxation time of 7.1 ms for 5.9 wt% carbonated hydroxylapatite indicates that the single line is likely due to rapid, high-symmetry jumps in translationally rigid D(2)O molecules, indicative of structural incorporation within the lattice. Discrimination between structurally incorporated and adsorbed water is enhanced by the rapid exchange of surface D(2)O with atmospheric H(2)O. Moreover, a (2)H resonance was observed for samples dried under a variety of conditions, including in vacuo heating to 150°C. In contrast, a sample heated to 500°C produced no deuterium resonance, indicating that structural water had been released by that temperature. We propose that water is located in the c-axis channels. Because structural water is observed even for apatites with very low carbonate content, some of the water molecules must lie between the monovalent ions.

Calcium orthophosphates: occurrence, properties, biomineralization, pathological calcification and biomimetic applications

The present overview is intended to point the readers' attention to the important subject of calcium orthophosphates. This type of materials is of special significance for human beings, because they represent the inorganic part of major normal (bones, teeth and antlers) and pathological (i.e., those appearing due to various diseases) calcified tissues of mammals. For example, atherosclerosis results in blood vessel blockage caused by a solid composite of cholesterol with calcium orthophosphates, while dental caries and osteoporosis mean a partial decalcification of teeth and bones, respectively, that results in replacement of a less soluble and harder biological apatite by more soluble and softer calcium hydrogenphosphates. Therefore, the processes of both normal and pathological calcifications are just an in vivo crystallization of calcium orthophosphates. Similarly, dental caries and osteoporosis might be considered an in vivo dissolution of calcium orthophosphates. Thus, calcium orthophosphates hold a great significance for humankind, and in this paper, an overview on the current knowledge on this subject is provided.

Effects of nanocrystalline calcium deficient hydroxyapatite incorporation in glass ionomer cements

Glass ionomer cements (GICs) are clinically attractive filling materials often employed in the field of dentistry as restorative and luting materials. The present work aims to formulate bioactive nanocrystalline calcium deficient hydroxyapatite (nCDHA)-GIC composite cements with improved mechanical and resorption properties of the set cement than GICs. The nCDHA was synthesized via an accelerated microwave process and characterized by X-ray powder diffraction (XRD) and Fourier transform infrared spectroscopy (FT-IR) methods. The synthesized nCDHA was mixed with GIC in different compositions (5, 10 and 15 wt%) maintaining the powder to liquid ratio. Cylinders of dimensions 8 mm height and 4 mm diameter were formed using a Teflon mold following a conventional cement forming technique. The XRD and FT-IR of the cylinders showed increased intensity and characteristic bands of CDHA with increase in nCDHA content. The surface cracks and the elemental composition of the set cements were analyzed by scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDX). Decreased surface hardness was observed for composite cements with increase in nCDHA addition. The cement cylinders were tested for ionic release in Millipore water (pH=7) via inductive coupled plasma (ICP) spectroscopy and in demineralization solution of pH=5 to find out the weight loss in an acidic environment at 37 °C performed periodically for 5 weeks. The ionic release percentage, weight loss and compressive strength were observed to increase with an increase in nCDHA addition.

Hydroxyapatite microparticles as feedback-active reservoirs of corrosion inhibitors

This work contributes to the development of new feedback-active anticorrosion systems. Inhibitor-doped hydroxyapatite microparticles (HAP) are used as reservoirs, storing corrosion inhibitor to be released on demand. Release of the entrapped inhibitor is triggered by redox reactions associated with the corrosion process. HAP were used as reservoirs for several inhibiting species: cerium(III), lanthanum(III), salicylaldoxime, and 8-hydroxyquinoline. These species are effective corrosion inhibitors for a 2024 aluminum alloy (AA2024), used here as a model metallic substrate. Dissolution of the microparticles and release of the inhibitor are triggered by local acidification resulting from the anodic half-reaction during corrosion of AA2024. Calculated values and experimentally measured local acidification over the aluminum anode (down to pH = 3.65) are presented. The anticorrosion properties of inhibitor-doped HAP were assessed using electrochemical impedance spectroscopy. The microparticles impregnated with the corrosion inhibitors were introduced into a hybrid silica-zirconia sol-gel film, acting as a thin protective coating for AA2024, an alloy used for aeronautical applications. The protective properties of the sol-gel films were improved by the addition of HAP, proving their applicability as submicrometer-sized reservoirs of corrosion inhibitors for active anticorrosion coatings.

Calcium Orthophosphates in Nature, Biology and Medicine

The present overview is intended to point the readers’ attention to the important subject of calcium orthophosphates. These materials are of the special significance because they represent the inorganic part of major normal (bones, teeth and dear antlers) and pathological (i.e. those appearing due to various diseases) calcified tissues of mammals. Due to a great chemical similarity with the biological calcified tissues, many calcium orthophosphates possess remarkable biocompatibility and bioactivity. Materials scientists use this property extensively to construct artificial bone grafts that are either entirely made of or only surface-coated with the biologically relevant calcium ortho-phosphates. For example, self-setting hydraulic cements made of calcium orthophosphates are helpful in bone repair, while titanium substitutes covered by a surface layer of calcium orthophosphates are used for hip joint endoprostheses and as tooth substitutes. Porous scaffolds made of calcium orthophosphates are very promising tools for tissue engineering applications. In addition, technical grade calcium orthophosphates are very popular mineral fertilizers. Thus ere calcium orthophosphates are of great significance for humankind and, in this paper, an overview on the current knowledge on this subject is provided.

Effect of Amine Additives on the Morphology of Hydroxyapatite

Long HA whiskers with uniform morphology, high aspect ratio, and good crystallinity could be prepared using a hydrothermal technique at 180 oC for 10 h. Whisker morphology was sensitively-dependent on preparation conditions. Compared with formamide and urea, acetamide showed a low hydrolysis rate; it was found to be an appropriate additive to synthesize HA whiskers. Length and aspect ratio could be controlled by selecting solution conditions with the Ca/P ratio at 1.67 in the range 42 - 84 mmol/L calcium and 25 - 50 mmol/L phosphate and 0.75 – 1.25 mol/L additive concentration (as amide, -NH2).

Rietveld refinements and spectroscopic structural studies of a Na-free carbonate apatite made by hydrolysis of monetite

Seven nominally identical samples of Na-free carbonate apatite (CO(3)Ap) were prepared by reaction of CaHPO(4) with ammonium carbonate solution at 70 degrees C over 3 days. They were studied by chemical analysis, Rietveld analysis of powder X-ray diffraction (XRD) data, Ca/P ratio determinations (quantitative phase analysis of CaO, Ca(OH)(2) and hydroxyapatite formed after heating to 900 degrees C from Rietveld analysis of XRD data), He pycknometry, (1)H, (13)C and (31)P MAS NMR spectrometry and Fourier transform infrared and Raman spectroscopy. Spectroscopy showed the apatite products were B-type CO(3)Aps (CO(3)(2-) replacing PO(4)(3-)) and XRD that one sample contained 1.6 wt% calcite with a trace in another. Mean results of the six essentially calcite-free samples were: a=9.405(5)A, c=6.896(2)A; 11.2 wt% CO(3); unit cell contents, Ca(8.241)(PO(4))(4.344)(CO(3))(1.656)(OH)(0.139) x 2.29H(2)O; mole Ca/P ratio from chemical analyses, 1.897(22) and from powder XRD phase analysis of samples decarbonated at 900 degrees C, 1.892(25). Density determinations indicated that the 2.29mol of H(2)O were in the unit cell. Rietveld refinements were undertaken without and with explicit modelling of the CO(3)(2-) ion. The latter used constraints to maintain the CO(3)(2-) ion in its known geometry and the total of PO(4)(3-) and CO(3)(2-) ions per unit cell at six. Without the CO(3)(2-) ion in the model, PO(4) volume, P-O bond lengths and P occupancy were apparently reduced, consistent with CO(3)(2-) replacing PO(4)(3-) ions. With the CO(3)(2-) ion modelled, the reductions were less and the CO(3)(2-) ion occupied the "sloping" face of the replaced PO(4)(3-) ion in two-fold disorder about the mirror plane. The angle between the normal to the plane of the ion and the c-axis was 34 degrees , close to 35.3 degrees , the equivalent angle for the PO(4)(3-) ion. When modelled, the CO(3)(2-) ion occupancy was 1.81 ions per unit cell, in reasonable agreement with unit cell contents determined chemically (1.66). The OH(-) ion occupancy was elevated (2.33 ions per unit cell versus 0.14 inferred from the charge balance), which we ascribe to H(2)O molecules occupying sites in c-axis channels. The Ca/P ratio from occupancies (2.31) was also elevated over that determined chemically (1.90). We attribute this to loss of Ca from Ca sites increasing the apparent anisotropic displacement parameters of remaining Ca atoms, leading to an apparently increased occupancy.

XRD Analysis of Calcium Phosphate and Biocomposite Calcium Phosphate/Bioresorbable Polymer

Composite biomaterials based on calcium phosphate ceramic due to their high bioactivity are of interest for biological application and bone tissue repair. Structural and microstructural parameters of inorganic constituent of these materials are very important for the synthesis and characterization of composites. Quantitative and qualitative content, crystallite size of phases, as well as the degree of crystallinity have a great influence on the quality of composites, their application and bone tissue repair. X-ray diffractometry was employed to investigate the components of biocomposite materials, calcium phosphate (CaP) ceramic and poly-DL-lactide-coglycolide (DLPLG) polymer, as well as the biocomposite obtained from the mentioned components. Composite biomaterial was obtained by modified emulsion process. Using the Rietveld refinement, we analyzed CaP as an inorganic component of the composite, whence we have determined structural and microstructural properties of ceramic component of the investigated composite. The results obtained by structure refinement show that calcium phosphate ceramic materials synthesized at room temperature contain hydroxyapatite HAp as a predominant phase. The calculated Ca/P ratio is 1.667. The Rietveld analysis revealed lattice parameters a(Å)=9.4324(7) and c(Å)=6.8785(6) that are in agreement with the theoretical values.

A Molecular Dynamics Study of the Thermodynamic Properties of Calcium Apatites. 1. Hexagonal Phases

Structural and thermodynamic properties of crystal hexagonal calcium apatites, Ca10(PO4)6(X)2 (X = OH, F, Cl, Br), were investigated using an all-atom Born-Huggins-Mayer potential by a molecular dynamics technique. The accuracy of the model at room temperature and atmospheric pressure was checked against crystal structural data, with maximum deviations of ca. 4% for the haloapatites and 8% for hydroxyapatite. The standard molar lattice enthalpy, delta(lat)H298(o), of the apatites was calculated and compared with previously published experimental results, the agreement being better than 2%. The molar heat capacity at constant pressure, C(p,m), in the range 298-1298 K, was estimated from the plot of the molar enthalpy of the crystal as a function of temperature, H(m) = (H(m,298) - 298C(p,m)) + C(p,m)T, yielding C(p,m) = 694 +/- 68 J x mol(-1) x K(-1), C(p,m) = 646 +/- 26 J x mol(-1) x K(-1), C(p,m) = 530 +/- 34 J x mol(-1) x K(-1), and C(p,m) = 811 +/- 42 J x mol(-1) x K(-1) for hydroxy-, fluor-, chlor-, and bromapatite, respectively. High-pressure simulation runs, in the range 0.5-75 kbar, were performed in order to estimate the isothermal compressibility coefficient, kappaT, of those compounds. The deformation of the compressed solids is always elastically anisotropic, with BrAp exhibiting a markedly different behavior from those displayed by HOAp and ClAp. High-pressure p-V data were fitted to the Parsafar-Mason equation of state with an accuracy better than 1%.

Constitutive modeling of the densification and the grain growth of hydroxyapatite ceramics

In the present work, constitutive models for densification and grain growth were employed to investigate the sintering behavior of pure hydroxyapatite ceramics. For densification study, lattice diffusion, grain-boundary diffusion, and interface reaction mechanisms, and for grain-growth study, surface diffusion mechanism, were considered respectively. Hydroxyapatite ceramics were pressurelessly sintered. The sintering results were discussed and compared with the modeling results. Based on the constitutive models employed and the experimental results obtained, grain-boundary diffusion was identified as the dominant mechanism for the densification of the investigated hydroxyapatite. The grain-growth model provided a good prediction to the grain growth of the investigated hydroxyapatite. The activation energies for densification and grain growth of hydroxyapatite ceramics were evaluated as 1150+/-40 and 1020+/-40 KJ mol(-1), respectively.

Rietveld refinements and spectroscopic studies of the structure of Ca-deficient apatite

Nine samples of Ca-deficient apatite (Ca-def Ap) were prepared from suspensions of CaHPO4 (monetite) at 90 degrees C by raising the pH from approximately 4 through release of NH3 produced by the hydrolysis of urea. Products were dried at 100 degrees C for 24h and studied by chemical analyses, X-ray powder diffraction (XRPD) (and Rietveld analysis of this data), Ca/P ratio determination (quantitative phase analysis of samples after heating to 900 degrees C from Rietveld analysis of XRPD data), scanning electron microscopy, He pycknometry, 1H and 31P MAS NMR spectrometry and Fourier transform infrared and Raman spectroscopy. All samples contained apatite, but three also contained monetite. Infrared and Raman spectroscopy confirmed the presence of HPO4(2-) and absence of carbonate ions in the six monetite-free samples. Mean results for the six samples were: a = 9.4320(40), c = 6.8751(31) A; unit cell formula from chemical analysis neglecting protonation of phosphate ion, Ca(9.303(50))(PO4)6(OH)(0.606(99)).1.97(12)H2O; theoretical density 3.10 g cm(-3); experimental density (mean for three samples) 3.15 g cm(-3); and Ca/P mole ratio from chemical analysis and phase analysis after heating to 900 degrees C, 1.550(8) and 1.550(2), respectively. An earlier assignment of a line at 6 ppm in the 1H NMR spectrum of similar samples to HPO4(2-) ions could not be confirmed; hence no information about the HPO4(2-) ion content could be derived, in disagreement with the previous NMR study. A shoulder at approximately 0.9 ppm relative to 85 wt% H3PO4 in the 31P NMR spectrum was assigned to HPO4(2-) ions. Occupancies from the Rietveld structure refinements indicated preferential loss of Ca from Ca2 sites compared with Ca1, but the loss was substantially smaller than expected from chemical analyses. It is suggested that imperfect modelling of the structure in the refinement, particularly disorder associated with the Ca2 site, resulted in errors in Ca2 occupancies. The P-O bonds were slightly shorter than those in stoichiometric hydroxyapatite, rather than longer as might be expected from protonation of phosphate tetrahedra. However, consideration of known acid phosphate structures indicated that it was unlikely that the increase in P-O lengths would be sufficient to be detected. The observed decrease was tentatively assigned to the presence of Ca2+ ion vacancies.

Photocatalytic degradation of formaldehyde containing wastewater from veterinarian laboratories

The photocatalytic destruction of methanol, formaline (mixture of formaldehyde, methanol and water) and formaline wastes from the preservation of vertinarian physiologic samples has been attempted by two different processes, at high concentrations of reagents and by dossification of reagents, varying pH in both. Experiment evolution has been monitored by measuring the organic matter such as TOC and formaldehyde concentrations [H2CO]. Also, methanol and methanol-formaldehyde interactions with the TiO2 surface have been analysed by FTIR spectroscopy. Results indicate that at high concentrations the catalyst surfacial alterations given by methoxy, formates or carbonates, according to the pH of the sample can profoundly affect catalyst behaviour. It has been established that reagent dossification is advantageous for enhancing photonic efficiency as it minimizes the adsorbate presence that hampers the photocatalytic process.