Amelioration Strategies for Silver Diamine Fluoride: Moving from Black to White

Topical cariostatic agents have become a reasonable alternative for managing dental caries in young children. Silver diamine fluoride (SDF) is a practical topical approach to arrest caries and avoid extensive and risky dental treatment. However, the literature demonstrates a parental hesitation towards accepting SDF because of black unaesthetic tooth discolouration following application. The rapid oxidation of ionic silver darkens demineralised tooth structure permanently. In this regard, nano-metallic antimicrobials could augment or substitute for silver, and thereby enhance SDF aesthetic performance. Recently, biomedical research has drawn attention to selenium nanoparticles (SeNPs) due to their antimicrobial, antioxidant, and antiviral potencies. Various in vitro studies have examined the effect of SeNPs on the virulence of bacteria. This narrative review explores practical issues when using SDF and suggests future directions to develop it, focusing on antimicrobial metals. Several methods are described that could be followed to reduce the discolouration concern, including the use of nanoparticles of silver, of silver fluoride, or of selenium or other metals with antimicrobial actions. There could also be value in using remineralising agents other than fluoride, such as NPs of hydroxyapatite. There could be variations made to formulations in order to lower the levels of silver and fluoride in the SDF or even to replace one or both of the silver and fluoride components completely. Moreover, since oxidation processes appear central to the chemistry of the staining, adding SeNPs which have antioxidant actions could have an anti-staining benefit; SeNPs could be used for their antimicrobial actions as well. Future research should address the topic of selenium chemistry to optimise how SeNPs would be used with or in place of ionic silver. Incorporating other antimicrobial metals as nanoparticles should also be explored, taking into account the optimal physicochemical parameters for each of these.


Introduction
Globally, one of the most widespread childhood illnesses is Early Childhood Caries (ECC). Statistically, this represents some 48% of chronic childhood diseases [1], and is the major reason for preventable hospital admissions in children. Given that it affects children who are too young to cooperate with conventional dental treatment, ECC is typically treated invasively under general anaesthesia (GA). From an economic point of view, untreated ECC is a significant burden to the healthcare system. According to a recent study, a single dental GA session costs over USD 1000 (AUD 1793.23 with SD 803.45) [2], and there is often a long GA waitlist for public sector dental treatment. Private treatment under GA is not affordable for many parents. Hence, there is great value in alternative management options to treat and arrest ECC in young pre-cooperative children that can be quick, simple, painless, affordable, effective, and easy to apply.
In recent years, silver diamine fluoride (SDF) has emerged as a popular topical treatment for achieving caries arrest in young children, thereby reducing the need for invasive and costly restorative or surgical dental treatment [1]. SDF is composed of silver, ammonia, Lately, selenium nanoparticles (SeNPs) have been attracting interest in biomedical research owing to their suitable biocompatibility as well as their antibacterial, antifungal, antiviral, and antioxidant effects [18]. SeNPs have been shown to exert antimicrobial actions against multiple species of bacteria [19]. The antibacterial and antioxidant effects of SeNPs in topical anticaries agents has not been investigated thoroughly [19]. The specific aims of this review are to (1) explore how the chemical composition of SDF could be altered to reduce staining and (2) explore the possibility of using alternative nanomaterialbased antimicrobial agents to augment or replace ionic silver, thus lowering the potential to cause black discolouration of teeth.

Early Childhood Caries: The Ripple Effect
ECC is one of the most common chronic disorders in the world [20]. ECC is defined as "the presence of one or more decayed (non-cavitated or cavitated lesions), missing (due to caries), or filled tooth surfaces in any primary tooth in a child under the age of six" [21]. It is challenging to provide restorative treatment for younger children because of their immaturity as well as their inability and unwillingness to cooperate for treatment by dentists and therapists provided in the dental chair. Recently, the COVID-19 pandemic crisis has worsened the situation regarding caries in children because parents have postponed their children's dental appointments [22]. Moreover, in communities that are facing major public health issues because of social disadvantage, high rates of caries in the primary dentition may not be a high priority compared with other health needs [23,24].
ECC affects a child's life in multiple ways, causing pain, infection, nutrition difficulties, interrupted sleep, self-esteem concerns, and aesthetic problems [25,26]. Advanced cases of ECC can cause further complications such as impaired dietary intake, school absence, impaired growth and development, and repeated emergency admissions and hospitalisations for severe dental infections. Due to the cognitive and communication problems of treating preschool and very young children, dental treatment under GA is often needed. In Australia, there is a growing need for paediatric dental GA, and the waitlist in some public healthcare facilities may extend up to two years [27]. Children who undergo dental GA for the treatment of ECC have a reported relapse rate of up to 79%, as they continue to develop new carious lesions and symptoms requiring repeated treatment [28].

Early Childhood Caries: The Ripple Effect
ECC is one of the most common chronic disorders in the world [20]. ECC is defined as "the presence of one or more decayed (non-cavitated or cavitated lesions), missing (due to caries), or filled tooth surfaces in any primary tooth in a child under the age of six" [21]. It is challenging to provide restorative treatment for younger children because of their immaturity as well as their inability and unwillingness to cooperate for treatment by dentists and therapists provided in the dental chair. Recently, the COVID-19 pandemic crisis has worsened the situation regarding caries in children because parents have postponed their children's dental appointments [22]. Moreover, in communities that are facing major public health issues because of social disadvantage, high rates of caries in the primary dentition may not be a high priority compared with other health needs [23,24].
ECC affects a child's life in multiple ways, causing pain, infection, nutrition difficulties, interrupted sleep, self-esteem concerns, and aesthetic problems [25,26]. Advanced cases of ECC can cause further complications such as impaired dietary intake, school absence, impaired growth and development, and repeated emergency admissions and hospitalisations for severe dental infections. Due to the cognitive and communication problems of treating preschool and very young children, dental treatment under GA is often needed. In Australia, there is a growing need for paediatric dental GA, and the waitlist in some public healthcare facilities may extend up to two years [27]. Children who undergo dental GA for the treatment of ECC have a reported relapse rate of up to 79%, as they continue to develop new carious lesions and symptoms requiring repeated treatment [28]. Dental GA sessions are a major financial burden for the health care system. According to the National Independent Hospital Pricing Authority, the average direct cost for a typical ECC treatment episode (including extractions and restorations) under GA was AUD 3029 in 2012-2013. Additionally, a retrospective study (from 2018-2019) reported that dental extraction under GA was the most frequent treatment choice when young children presented to emergency departments with severe dental infections [29].
Dental caries is a complex disease that involves multiple microbes (both bacteria and fungi) which emerge due to ecological changes in the dental plaque biofilm. This dysbiosis is driven by diet and other lifestyle factors [30]. There is an imbalance between acidogenic and aciduric microbes on one hand, and health-associated commensal bacteria on the other [31]. Even though dental caries is polymicrobial in nature, Streptococcus mutans Antibiotics 2023, 12, 298 4 of 18 (S. mutans) serves as a key pathogen by initiating the formation of a dense extracellular polymer matrix composed of glucan-rich exopolysaccharides. S. mutans and Streptococcus sobrinus (S. sobrinus) can both produce large quantities of these insoluble glucans [32], which give the biofilm enhanced bulk and adhesiveness and serve as a source of fermentable carbohydrates [33,34]. Therefore, impairing the growth and metabolism of these bacteria is desirable.

Silver Diamine Fluoride SDF: Sharpening an Anticaries Agent
As traditional approaches to the management of caries in young children are challenging, expensive, and often risky, there has recently been a marked shift towards minimally invasive approaches. SDF, as a topically applied antimicrobial agent, leverages the antimicrobial actions of several components: the alkaline pH (pH range from 9-13), the silver ions (25% by wt), fluoride ions (5% by wt), and ammonia (8% by wt). The balance of 62% is water, which acts as the solvent [35].
SDF was first introduced in Japan in the 1960s by Nishino, who added ammonia to improve on existing silver fluoride formulations [36]. While marketed with regulatory approval for treating dentinal hypersensitivity, SDF is used widely off-label for arresting dental caries in both deciduous and permanent teeth [37]. As a topical agent, it is more effective than fluoride varnishes [3]. SDF has demonstrated greater efficacy for halting the progress of lesions and gives increased fluoride absorption into tooth structure when compared with fluoride varnishes and topical fluoride gels, in both in vitro and in vivo studies [38,39].
The effectiveness of SDF in causing caries arrest is attributed to the combined actions of the high concentrations of fluoride and silver ions as well as the alkaline pH [40]. Ionic silver attacks bacterial microbes via multiple approaches, as explained in detail below. Moreover, the silver ions in SDF form a layer of silver phosphate that provides resistance to further decay, while fluoride ions convert hydroxyapatite to less soluble fluorapatite [41]. Together, these actions inhibit the further progression of demineralisation and help to preserve dentine collagen, protecting it from further degradation [42].
SDF has been produced commercially in various concentrations (12%, 30%, 38%, and 40%), with the 38% versions being in widespread use [4]. These formulations contain 44,800 ppm of fluoride, which is 8 times above the threshold needed for antimicrobial actions on bacteria [4]. When applied to primary teeth, 38% SDF has greater potency than 12% SDF for caries arrest [5,43]. Worldwide, multiple SDF brands are manufactured. Advantage Arrest TM has a pH of 10 and contains 24.4-28.8% silver (w/v) and 5.0-5.9% fluoride. SDI Riva Star TM has a pH of 13 and contains 35-40% (w/v) silver fluoride (AgF) and 15-20% (w/v) ammonia. The manufacturer also provides a solution of potassium iodide (KI) to be applied immediately onto the surface to scavenge precipitated silver [44]. The same manufacturer also has an ammonia-free neutral formulation (pH 7.4), SDI Riva Star Aqua™, with 40% AgF. Removing the ammonia was intended to prevent transient gingival/mucosal burns. Another notable product, CSDS, is made by Whiteley. This is also ammonia-free, and contains 40% AgF and 10% stannous fluoride (SnF 2 ). The SnF 2 acts as a reducer for excessive silver ions [45].
The use of silver fluoride in various forms can exert potent effects on microbial growth. A significant reduction in levels of S. mutans has been seen following the application of SDF onto an infected dentine surface in vitro [46]. A similar beneficial effect on impairing the growth of Lactobacilli [47] has also been seen in an in vitro study. The high concentration of fluoride ions (44,800 ppm) exerts a two-level action on bacteria. Firstly, it disrupts bacterial enzymes that regulate carbohydrate uptake and metabolism. Secondly, it impairs biofilm formation [47].
With regard to the interaction between SDF and tooth structure, there is some evidence that SDF remineralises dentine [38]. This occurs through the creation of silver phosphate and the deposition of calcium fluoride, both of which contribute to an increase in pH (from 5.5 to 9-13). The calcium fluoride, which is believed to remain on tooth surfaces via protein-Antibiotics 2023, 12, 298 5 of 18 based globules, can discharge fluoride ions in acidic mediums (cariogenic conditions) and serves as an effective medium-term fluoride reservoir [48]. The formation of fluorapatite with its reduced acid solubility lessens the impact of acids produced by the dental plaque biofilm [35]. Silver ions that react and bind with hydroxyapatite form a type of protection shield against future cariogenic attacks [42]. Additionally, hydrolytic collapse of dentine collagen is impaired via inhibition of proteolytic enzymes, including cathepsins (or cysteine cathepsins) and matrix metalloproteinases (MMPs) [49]. Thus, the effectiveness of SDF results from multiple pathways, a point that is very relevant when considering how to enhance the formulation or substitute other components for the silver.
The ionic silver in SDF creates several issues. Due to the alkaline pH of SDF, during application, the solution must be applied in very small amounts, and some manufacturers recommend that gingival protection be applied before SDF. Proper cotton roll isolation is important because of the risk of causing burns to skin or mucosa [50]. The fate of silver applied to deep lesions that are close to the dental pulp is a further point. In the 1990s, Gotjamanos et al. [51] reported the possibility of silver ions penetrating through the tooth structure and reaching the pulp chamber. This situation would occur when SDF was applied to very deep carious lesions that were close to the dental pulp [52]. A further point is that SDF has a noticeable, odd metallic taste that may cause momentary nausea [7].
Going beyond these short-term issues, the most significant drawback of using SDF, especially in high concentrations, is that it creates long-lasting black stains on the treated surfaces due to the formation of silver compounds, especially silver phosphate and silver sulphide [53]. Within two minutes of SDF application, the treated dentine surface darkens noticeably and irreversibly. This is followed by a gradual increase in the intensity of staining from 5 min to 5 h post-application. The maximum colour change following SDF application occurs at 12 h. The intensity of the stains varies according to the frequency of application [6]. Paradoxically, the discolouration has been considered an SDF success indicator, since it represents a zone high in calcium, phosphorus, fluoride, and silver ions [38].
While parents might accept SDF staining on primary posterior teeth, they have lower acceptance of its application to primary anterior teeth, as these are within the aesthetic zone [9]. They may accede to its use on anterior teeth as a fall-back measure when the child displays an uncooperative attitude during regular dental visits [54,55]. In contrast, parents of a cooperative child are more inclined to prefer aesthetic tooth-coloured restorations for the anterior teeth of their child in order to alleviate the sense of guilt from not having managed their child's oral hygiene, and because reliable restorative techniques and procedures exist [56].
The black discolouration caused by SDF is an issue that could be addressed by using less silver or by using a scavenging agent. As mentioned previously, one manufacturer includes a topical application of saturated potassium iodide [57]. The rationale behind this is that the reaction between available silver ions and KI results in the precipitation of silver iodide (a bright yellow chemical compound) [58]. Silver iodide formation should minimise the concentration of free silver ions that eventually discolour the tooth surface. Despite this approach having been rated with "insufficient clinical evidence" in systematic reviews [59], in vitro studies have reported that the use of the SDF followed by KI application may lead to better aesthetic outcomes [60]. However, these outcomes appear to be temporary. A randomised clinical trial (RCT) showed no significant difference between the SDF/KI group and the SDF-only group at 30 months follow up [61]. They concluded that the further decomposition of silver iodide due to its photosensitivity leads to further release of silver and iodine. There is a concern that immediate application of KI following SDF may stain demineralised dentin [62]. This has prompted the search for a future antimicrobial agent that can achieve a balance between providing an adequate antibacterial effect and not significantly staining the tooth surface. Alternative potential antimicrobial agents include silver nanoparticles (AgNPs), selenium nanoparticles, and copper nanoparticles.
Nanoparticles (NPs) have diameters ranging from 1 to 100 nm. Their large surface area and strong chemical reactivity are desirable features [63]. Several metal-based nano-Antibiotics 2023, 12, 298 6 of 18 materials are being actively considered for inclusion in dental materials and therapeutic products [14]. Furthermore, combining NPs with polymers and coating them with other nanocomposites provides an opportunity for multiple types of physio-chemical modification [64]. Each modified form can have unique chemical and antibacterial properties. A high level of antibacterial activity is an anticipated result of the strong interactions between certain metal NPs and the negatively charged surface of bacterial cells. These interactions occur because of the vast surface area and high charge density of NPs [65].
As the size of NPs reduces, their antibacterial properties improve. For instance, 10 nm AgNPs are small enough to penetrate the bacterial matrix and can cause an imbalance in vital cellular functions like DNA replication, particularly in Gram-negative bacteria [2]. Moreover, by triggering oxygen radical production, they cause lipid peroxidation, which disrupts bacterial cell membranes and decreases bacterial metabolism [66]. There have been some preliminary studies of AgNPs. Tirupathi and others assessed the anticariogenic capacity of AgNPs loaded into a sodium fluoride (NaF) varnish and compared this with 38% SDF [67]. It proved to be as effective as the SDF but without causing undesirable postoperative staining [67]. Additionally, the AgNPs did not generate silver oxide when exposed to oxygen in the medium; hence, the demineralised enamel did not stain black [68]. Furthermore, in an in vitro study, Targino and others compared 38% SDF and a nano form of silver fluoride, finding that AgNPs exert greater antimicrobial actions than silver ions [69]. This provides some support for the extension of this line of work. Likewise, the further development of hydroxyapatite NPs might result in the replacement of fluoride in topical anticaries agents, thusavoiding issues with fluoride in young patients [2].

Silver: The Old New
For thousands of years, the antimicrobial actions of silver have been recognised. In 335 BC, Alexander the Great used silver vessels for storing water and silver jugs for drinking [70]. The action of silver in keeping water from fouling was recognised before the advent of the "Germ Theory of Disease" [71]. Silver compounds were used in wound healing remedies by ancient Greeks due to their antimicrobial properties [72], and this idea now extends into the modern age to dressings for burns. Prior to the advent of antibiotics, silver was a widely used antibacterial agent [73].
In dentistry, silver nitrate (AgNO 3 ) was used as a caries-arresting solution and an antibacterial agent since the 18th century [74]. In the 1840s, AgNO 3 was used to treat caries in primary teeth and for control of gingival disease, marking the first documented use of silver compounds in dentistry. Thereafter, it was further used as a dentine desensitiser, cavity disinfectant, and caries preventive treatment for permanent molars [75][76][77]. In 1908, GV Black described using AgNO 3 to treat caries in children as "the first measure against the disease" [78]. Then, in the 1960s, it was proposed that silver and fluoride should work together to prevent cavities; however, because of the discolouration of caries lesions, therapeutic use of silver fluoride compounds was limited [3].
Several modes of action of silver explain its antibacterial actions. Firstly, it attacks bacterial cell walls and changes their permeability, which leads to the leakage of cellular contents and impaired transport activity across the membrane [79]. Silver also generates reactive oxygen species (ROS) and hydroxyl radicals, which damage microbial DNA and cause lipid destruction through oxidation [80]. Ionic silver also inhibits the proton motive force, leading to apoptosis [81]. Moreover, AgNPs deactivate respiratory chain dehydrogenases, which inhibits cell respiration and growth. Once combined with halides, AgNPs release silver ions and produce more ROS [82]. A recent study by Hovhannisyan and colleagues pointed out that the effect of green synthesised AgNPs on bacterial membrane permeability was due to effects on formate hydrogenlyase proton-potassium transporters, energy-dependent H + -fluxes, and H + -translocating ATPase activity [83]. In addition, by specifically disturbing F O F 1 -ATPase activity, these biogenic AgNPs can affect bacteria that have no respiratory chain such as the Enterococcaceae species. The antimicrobial mechanisms of action of AgNPs are demonstrated in Figure 2.
leagues pointed out that the effect of green synthesised AgNPs on bacterial membrane permeability was due to effects on formate hydrogenlyase proton-potassium transporters, energy-dependent H + -fluxes, and H + -translocating ATPase activity [83]. In addition, by specifically disturbing FOF1-ATPase activity, these biogenic AgNPs can affect bacteria that have no respiratory chain such as the Enterococcaceae species. The antimicrobial mechanisms of action of AgNPs are demonstrated in Figure 2.

SDF Re-Composition: Nanometals vs. Ionic Silver
Although the individual chemical ingredients of SDF work in cooperation to halt caries, the question arises as to whether the same or better clinical actions would be achieved with less staining by reformulation of the product. Hence, several research groups have evaluated the use of nano formulated approaches instead of traditional silver-based topical cariostatic agents [63,68,[84][85][86]. Nano-silver fluoride is one of the suggested substitutes [63]. A study conducted by Nagireddy et al. used chemically synthesised AgNPs added to 0.05 ppm NaF and tested this formula for anticaries effects on 100 primary teeth. The results of their study showed 78% caries arrest within seven days. However, an issue with the design of their study was that normal saline was used as a control treatment, and there was no direct comparison to SDF [63].
In another study, SDF was modified by adding different concentrations of copperdoped bioglass nanoparticles (CuBGNPs), and the mixtures were assessed for their viscosity and antibacterial actions [84]. This modified form of SDF showed improved ion release and decreased cytotoxicity; in addition, a cumulative increase in the antimicrobial

SDF Re-Composition: Nanometals vs. Ionic Silver
Although the individual chemical ingredients of SDF work in cooperation to halt caries, the question arises as to whether the same or better clinical actions would be achieved with less staining by reformulation of the product. Hence, several research groups have evaluated the use of nano formulated approaches instead of traditional silver-based topical cariostatic agents [63,68,[84][85][86]. Nano-silver fluoride is one of the suggested substitutes [63]. A study conducted by Nagireddy et al. used chemically synthesised AgNPs added to 0.05 ppm NaF and tested this formula for anticaries effects on 100 primary teeth. The results of their study showed 78% caries arrest within seven days. However, an issue with the design of their study was that normal saline was used as a control treatment, and there was no direct comparison to SDF [63].
In another study, SDF was modified by adding different concentrations of copperdoped bioglass nanoparticles (CuBGNPs), and the mixtures were assessed for their viscosity and antibacterial actions [84]. This modified form of SDF showed improved ion release and decreased cytotoxicity; in addition, a cumulative increase in the antimicrobial effect was observed as the concentration of CuBGNPs rose. Discolouration was not investigated.
The combination of fluoride ions and silver NPs has also been compared with SDF [86]; the two provide similar bactericidal effects, but the version with AgNPs does not cause noticeable tooth discolouration. Taken together, these results suggest that the use of nanoparticles of silver or of silver fluoride could be one way to solve the problem of discolouration.

Selenium: A Plausible Substitute
Selenium (Se) was identified as a natural element by the Swedish chemist Jon Jakob Berzeliusin 1817 [87]. It occurs naturally in limited amounts. Most commercial Se is refined from other metals such as copper (Cu) using electrolysis [88]. Se can be organic (selenocysteine and selenothionine) or inorganic (selenate and selenite). Based on its configuration, it can exist either in a crystal form (monoclinic and trigonal) or in an amorphous polymorphic form [89]. Se is one of the group-6 elements in the periodic table and is situated between non-metallic sulphur (S) and metallic tellurium (Te) in the oxygen group [90]. Thus, the chemistry of Se is somewhat similar to that of sulphur, although the two have different oxidation capacities [91]. Se shows multiple oxidation states: Se 2+ , Se 4+ , Se 6+ , and Se 2-18 .

Selenium and Human Health
Selenium is an essential micronutrient in the human diet. Generally, Se has a valuable impact on human health at different levels. It plays a vital role in several intracellular activities [92], and its sufficient intake is linked to protection against various diseases, including hypercholesterolemia, some malignancies, and cardiovascular illnesses [93]. Se has unique growth-regulating capabilities and fulfils a role as a cofactor for essential enzymes such as thioredoxin reductases (which control the redox state of the cell by inhibiting thioredoxin) and glutathione peroxidases (which prevent cellular destruction caused by oxygen-free radicals) [94]. Other health-related effects of Se include its antibacterial and antiviral actions [95]. As a trace element in the diet, Se is essential for human health [96]. Decreased Se levels have been correlated to impaired cognitive ability, weak immunity, and increased mortality risk [97].

Selenium Nanoparticle Properties: Size, Shape, and Synthesis
SeNPs have a core of inorganic Se (0). They can exert their own actions or can be loaded with additional therapeutic agents [98]. SeNPs have superior biosafety and biocompatibility to organic or inorganic Se molecules [99]. In addition, they exhibit distinctive chemical and physical characteristics owing to their large surface-to-volume ratio and high surface energy [100]. SeNPs have high bioavailability but low toxicity [101].
Additionally, the parameters of SeNPs can be adjusted to optimise their physical and chemical features, including through altering the synthesis process or by using capping agents [102]. SeNPs have been synthesised in a range of configurations, including spheres [103], cubes [104], ribbons [105], flowers [106], nanorods [107], and nanoneedles [108]. Generally, the most frequently used shape for therapeutic purposes is spherical [109]. Wire-shaped SeNPs have shown greater photoconductivity, while sphericalshaped SeNPs have demonstrated greater biocompatibility [110,111]. Furthermore, certain forms of SeNPs are generated by specific synthetic processes. For instance, green synthesis can result in SeNPs with a hexagonal ring form [112][113][114].
The adjustability of SeNPs extends to their charge. SeNPs are normally negatively charged, but this can be altered to positive by using surface modifiers such as chitosan [115,116]. This type of tuning is important in terms of binding to different types of microorganisms. In terms of the antimicrobial actions of SeNPs, one study reported that both Gram-positive and Gram-negative bacteria were inhibited to a similar extent by SeNPs [117]. In contrast, another study by Wang and others found that SeNPs were less effective against Gramnegative bacteria (P. aeruginosa and E. coli) after 72 h of treatment [118]. It is possible that SeNPs could be less active against Gram-negative bacteria due to electrostatic repulsion between negatively charged lipopolysaccharide in the bacterial cell membrane and SeNPs, unlike the situation with Gram-positive bacteria, which have a slight positive or neutral surface charge [119]. Although the antimicrobial impact of SeNPs has not yet been fully explored [120], this parameter has been demonstrated in several studies, and these are summarised in Table 1.

Selenium Nanoparticles and Toxicity Concerns
Although selenium is a trace element and is essential for human body, there is a fine line between healthy therapeutic levels of selenium and a harmful toxic concentration. As a supplement, the World Health Organisation recommends 55 µg of selenium per day for healthy individuals [121]. Selenium compounds, especially at high concentrations, exhibit noticeable toxicity. On the contrary, SeNPs have better safety use and cause less cytotoxicity [110]. In vivo evidence has shown that the SeNP lethal dose is 4-6 times less than other formulations of organic and inorganic selenium [122]. In general, the cytotoxic Antibiotics 2023, 12, 298 9 of 18 actions of SeNPs are closely associated with their ability to damage cell membranes and to generate oxidative stresses [123]. Furthermore, stabilisation of SeNPs could play a part in further reducing their toxicity [107]. For example, polysaccharide-stabilised SeNPs show less toxicity and enhanced bioavailability [110].
It has been reported that if human selenium consumption surpasses 400 µg/day, symptoms of selenosis (selenium intoxication) might be observed. Selenosis causes symptoms that range from hair and nail damage to severe neuromotor disturbances. However, other studies argued that a much higher intake (e.g., 3000 µg/day) is needed to cause symptoms of selenosis [123]. The median lethal dose of SeNPs is 92.1 mg/kg, while it is only 14.6 mg/kg for seleno-methylselenocysteine [101]. The cytotoxicity of SeNPs varied according to their synthesis method. Biogenic (green) synthesis results in less toxic forms of nano selenium compared with chemically synthesised SeNPs [124]. Lastly, in terms of genotoxic impacts of SeNPs, some studies have found that bare (uncoated) SeNPs cause a dosedependent genotoxic action; however, this problem can be reduced markedly by capping or coating SeNPs with surfactants [107]. Overall, in terms of clinical use under controlled conditions where little or no ingestion is likely to occur, concerns over the cytotoxic effects of SeNPs are likely to be low. Moreover, this concern can be minimised by using green synthesis and by applying suitable capping agents.

SeNPs and the Role of Capping Agents
Stabilisers/capping agents can be added to chemically synthesised SeNPs to enhance their physio-chemical characteristics and maintain their bioactivity and size by enhancing their stability and preventing them from agglomerating [125]. Furthermore, such stabilisation influences the ultimate size of SeNPs and, thus, is strongly related to antioxidant capacity, cytotoxicity, and antimicrobial actions [93]. Multiple agents have been used to coat SeNPs. Chitosan is a type of chitin that can be extracted from insects and crustaceans. Chitosan-capped SeNPs have reasonable bioavailability, a positive charge, and minimal toxicity; thus, they have been used in several therapeutic applications [126]. Various polymers have also been used to stabilise SeNPs, including polyvinyl alcohol, polyethylenimine, and polyvinylpyrrolidone [110]. Plant-derived polysaccharides have been used to cap SeNPs, and this enhances their antioxidant actions [127,128]. Proper selection of capping agents can be used to optimise the chemical, physical, and biological properties of selenium nanoparticles. Organo-selenium dental sealant was able to inhibit the growth of all species individually and in a mixed biofilm. As shown in Table 1, SeNPs possess microbial inhibition action against a variety of bacterial strains as well as against important fungi (C. albicans), and this action varies according to the synthesis method and the SeNP properties. Some studies have adopted ecofriendly green synthesis using plants, fruit, vegetables, and natural substances. Others have followed traditional chemical synthesis approaches and then used capping agents. Most studies of SeNPs began with Na 2 SeO 3 as the precursor molecule. A noteworthy pattern across these studies was that, regardless of the variables being examined, all the microbes used were affected by exposure to SeNPs to some extent. A limitation of the work to date is that most studies used organisms in the planktonic state rather than in biofilms; moreover, biofilm studies were single species rather than multispecies. Hence, additional work is needed to determine the optimum profile of SeNPs for achieving caries-arresting actions.

Conclusions and Future Directions
SDF has become a popular treatment choice for halting dental caries in young children, especially children with minimal cooperation for dental procedures. However, it darkens hard tooth structure permanently, which compromises aesthetics, particularly in the anterior smile zone. This review identifies several methods that could be followed to reduce this concern, including the use of AgNPs and NPs of silver fluoride. There could also be value in using remineralising agents other than fluoride, such as NPs of hydroxyapatite. There could be variations made to formulations in order to lower the levels of silver and fluoride in the SDF or even to replace one or both of the silver and fluoride components completely. Moreover, since oxidation processes appear central to the chemistry of the staining, adding SeNPs which have antioxidant actions could have an anti-staining benefit; SeNPs could be used for their antimicrobial actions as well. Future research should address the topic of selenium chemistry to optimise how SeNPs could be used with or in place of ionic silver. Incorporating other antimicrobial metals as nanoparticles such as copper, zinc oxide, and cerium oxide should also be deeply explored in the context of topical anticaries agents, taking into account the optimal physicochemical parameters for each of these.

Conflicts of Interest:
The authors declare no conflict of interest.