In our society the demand for healthy living is growing constantly. Therefore, the inter-est in materials which are able to control microorganism proliferation or even kill them selectively, is rising tremendously. Ordinary materials like door knobs or computer key-boards but also materials used in medicine (catheters, surgical instruments) or food packaging industry are not antibacterial itself. Surface modifications are needed to ad-dress this issue.
Inspired by nature, two main strategies exist to eliminate or reduce the extent of bacte-rial attachment: On the one hand chemical surface properties (e.g. hydrophobicity)209 and surface roughness10 are used to keep surfaces bacteria- free by coating correspond-ing polymers or metal oxide nanostructures onto the surface. These topography con-cepts operate directly on the interface of cell surface interaction and disrupt the coloni-zation of bacteria cells. The second approach copied from nature is the application of biocidal acting coatings.210 In this area, many agents have been explored to be inte-grated in the matrix or on the surface of biorelevant materials: antibiotics8, quaternary ammonium compounds11, 211, silver ions212 , enzymes213 or highly reactive small mole-cules214, 215.
The world-wide leading antibacterial coating material is silver in its various forms. The antibacterial activity of silver is known for hundreds of years but its success story is an-ything but linear and marked by many challenges. Silver nitrate was the first silver com-pound applied in a targeted manner for wound healing and infection curation in the 19th century.216 Another very prominent silver compound used as a broad-spectrum antibac-terial agent, was reported in 1968, named silver sulfadiazine, and possesses antifungal and antiviral activity.217 With the development and the introduction on the market in 1940, penicillin predominated the “antibacterial world” for decades. Hence, the use of silver for the treatment of bacterial infections decreased.218, 219 But based on the up-coming, increasing antibiotic resistance during the last years, silver returns back in the focus of scientific research.
The evolution of silver compounds as general antibiotic agent correlates strongly with the developments for silver in the field of material science. At the beginning of the 19th century, silver impregnating materials/dressings have been applied in a broad manner;
afterwards huge efforts have been made to combine antibiotic agents (e.g. penicillin) with carrier materials in order create antibacterial surfaces.220, 221 With the discovery of nanotechnology and therefore the targeted synthesis of silver NPs, silver as antibacterial coating was re-discovered.222 In this regard also the reaction mechanism of silver NPs has been investigated for a better understanding of antibacterial action. Still today the exact mechanism is not fully explained. But there exist two theories which are widely accepted. One theory is, that the antibacterial action is related to the amount of availa-ble silver ions, the so-called ion mediated killing.223 Silver in its elementary state is inert but when ionized (which occurs easily for example in the fluid of wounds) it binds to bacterial tissue proteins leading to structural changes and ending up with cell death.224,225 Furthermore Ag+ ionsbind to DNA and RNA of bacterial cells initiating dena-turation and inhibiting bacterial replication.216 The second explanation for the antibac-terial activity of silver is based on the contact killing concept. Silver NPs bind to thiol groups present in bacterial cells and therefore block the uptake of essential substrates (phosphates, amino acids or carbohydrates) and inhibit the respiration process which leads to cell death.226-228 Silver NPs are also discussed critically as undesired penetration in the human body can effect argyria.229 Therefore, the immobilization of silver NPs is highly important and is realized nowadays via different approaches: Embedding in poly-mers230, fibre glasses or hydrogels and binding onto materials like silica231 or carbon nanotubes232 reduces the uncontrolled risk of silver exposure into the environment. One prominent interaction of silver ions (Ag+) as well as metal silver NPs is the coordination towards SH functionalities. Therefore, SH-containing mesoporous silica materials exhibit a great potential for immobilizing silver substrates not least due to their large internal
surface area (up to 1000 m2/g).233 The loading of Ag NPs on thiol comprising, porous silica materials is realized via different synthetic pathways. Infiltration of Ag NPs or direct synthesis on the mesoporous silica materials are the main tasks.234 The uptake of silver components strongly depends on the quantity of accessible SH groups on the corre-sponding material. As post-synthetic and co-condensation approaches are mainly used to introduce surface active SH groups on silica, the Ag+ ion and Ag NP coordina-tion is limited by the funccoordina-tional degree of maximal 30 %. According to this scientific gap, PMO materials would represent the ideal platform to bind Ag+ ions/Ag NPs in a quantitative manner. In this connection enhanced (antibacterial) activity of silver com-ponents are expected.
Acidity as a tool to effectively kill bacteria, is not new.235 But the use of acidic materials as antibacterial surfaces is rather exceptional and only few reports in this direction have been published so far.236 To make surfaces acidic two concepts exist: the application of Lewis acids or the use of proton releasing Brønstedt acids. Representative Lewis acids are metal oxides which react acidic in contact with water. For example, Guggenbichler et al. present the concept of MoO3 to create germ-free surfaces.237 Brønstedt acids, which provide surface active protons, are represented through immobilized organic modification on corresponding surfaces. In this context sorbic acid coated films are eval-uated by Wunderlich et al. in 2011, to protect food packaging relevant surfaces.238 An-other very promising organic modification for the release of acidic protons is the sulfonic acid group (-SO3H). Nafion is one candidate of sulfonic acid containing material on poly-meric basis. Kim et al. reported the bacterial adhesion of several pathogenic bacteria strains on Nafion films with regard to physical surface properties.239 In this regard, it was already concluded in 2009 that Nafion possesses great potential in antibacterial surface design. But what they didn`t consider in detail, was the proton release of Nafion fol-lowed by upcoming acidity. Missing acidity studies are the consequence. Until now, a representative counterpart in silica chemistry for polymeric Nafion is still missing.240 The design of silica materials, having a very high density of sulfonic acid groups, not only on the surface but also over the entire bulk volume, is a very challenging issue even nowadays. A conceivable solution of this problem might be the use of single source PMO precursors carrying SO3H entities.
A great disadvantage of the system “nanosilver” and “acidity” is the missing control about Ag+ ion and acidic H+ release. Also the fact, that Ag NP carrying materials and pro-ton releasing materials are not long-term efficient as the material leaches out slowly, the demand for novel long-term active materials with controllable antibacterial activity rises.241 Recently the strategy of antimicrobial photodynamic therapy (aPDT) was estab-lished by Denis et al.242 The concept is inspired by the photodynamic therapy applied in cancer research since the 1970s.243, 244 Based on the use of immobilized non-toxic pho-toactive substances, namely photosensitizer (PS), visible light is absorbed and reactive oxygen species (ROS) are produced in presence of environmental O2. ROS formation oc-curs according to two pathways depending on available substrates. Type I describes the production of free organic radicals by an electron-transfer reaction of the excited state of the PS to organic substrates. Interacting with molecular oxygen these free organic radicals generate ROS such as hydrogen peroxide (H2O2), hydroxyl radicals (OH.) or su-peroxide radicals (O2.-).243, 245 In type II the production of singlet oxygen (1O2) is illus-trated by the reaction of triplet state PS with O2.246 These electronically, excited and highly reactive states of oxygen react with a large number of essential biological sub-strates inducing harmful damage on the cell membranes and cell walls. Cell death of microorganism such as bacteria and fungi is the final consequence.247, 248 The quality of the PS mainly contributes to the efficiency of aPDT. In literature three types of PS are discussed, illustrated in Fig. 7: Phenotiazine (compound 17), tetrapyrrole (compound 18) and coumarins (compound 19).249 Depending on the specific application area (environ-mental conditions; wavelength of light), the corresponding PS type is used. The inter-ested reader is referred to
PS, is shortly introduced as it belongs to one of the most effective PS under visible light illumination.251-254 Its absorption bands are located between 480 nm and 550 nm and its singlet oxygen production yield Φ is very high with Φ(1O2)= 0.75.251 Additionally, Rose Bengal possesses a fairly long lived triplet state (t1/2= 0.1-0.3 ms) and a triplet quantum yield of ΦT= 0.76 that leads to very effective ROS production rates.254-256
Fig. 7. Prominent classes of PS; (17) phenotiazine type, (18) tetrapyrrol type and (19) coumarine type.
The immobilisation of RB on materials represents an essential step in the aPDT as the goal is to guarantee long-term stability of surfaces against bacteria attachment and no leaching out of the dye. Furthermore, by binding or embedding RB on silica244 the self-quenching effect of RB decreases.257 This is attributed to the reduced presence of aggre-gated RB species which are responsible of lower singlet oxygen quantum yields. Accord-ing to Guo et al.258 and Gianotti et al.251 the attachment of RB on amine functionalized silica NPs (post-functionalized silica NPs with aminopropyl-trimethoxysilane) occurs via free COOH entities of RB. This procedure represents the standard immobilisation con-cept of RB on silica surfaces and is illustrated in Fig. 8.
Fig. 8. Standard method to immobilize RB (COOH fct. highlighted in red) on silica surfaces (amine fct.
highlighted in blue) via amide bound (purple).259
The disadvantage of this pathway is the resulting amide bound that is instable towards aqueous media where hydrolysis takes place. This property represents a significant drawback as the aPDT often takes place in aqueous media and thus RB is cleaved slowly.
Alternative techniques to create a stable connection between RB molecules and silica surfaces are highly requested.
Beside the concept of ROS production, another signalling molecule is in the focus of an-tibacterial action: nitric oxide (NO). Nitric oxide (NO) is very often considered to be just another signalling molecule in vivo. But in the last decade, NO is attracting increasing attention regarding the application in the biomedical field, named cancer therapy260 or as antibacterial agent261. Therefore, the design of NO hosting or NO donating materials is focused during the last years. Representative molecular modifications for NO binding are given by the following functional groups seen in Fig. 9: diazen-1-ium-1,2-diolates (diazeniumdiolates; compound 20), S-nitrosothiol (compound 21), metal-nitrosyl com-plex (compound 22) and nitrobenzene (compound 23).262 The release of radical NO oc-curs via different pathways depending on the NO donor. Diazeniumdiolates cleave NO
via an acid-catalysed reaction mecha-nism whereas metal-nitrosyl263 com-plexes and nitrobenzene264 compo-nents release NO by UV light irradia-tion. The most versatile NO donor is S-nitrosothiol as it has several NO re-lease mechanism: Heat, light or the presence of copper ions.265 A very de-tailed review about NO donors in con-nection with biological applications is given by Wang et al. and is not dis-cussed herein.266 The combination of
materials science and NO hosting is briefly summarized. Beside chitosan derivatives267 for NO binding also silica NPs268 are reported as NO-donor hosting materials. Firstly re-ported by Zhang et al. (2003), fumed silica NPs, post-modified with amine groups, have been applied by default.269 An absolute luminary in the field of NO binding using silica NPs is M. Schoenfisch who published about 20 publications only in the last 5 years (ac-cording to database web of science; Oct. 2015). His concept is based on non-porous silica NPs which are modified with diazeniumdiolate or S-nitrosothiol entities as NO do-nors.270, 271 It is very surprising that despite the evolution of dense packed silica NPs towards highly porous spheres, no efforts have been made to use porous silica NPs in the context of NO storage. One possible reason is the direct connection between the functional degree of SH groups on the surface (limited to 25 % using co-condensa-tion/post-synthetic approaches) and NO storage efficiency. As already the non-porous silica systems show significant antibacterial efficiencies, imagine the effects if quanti-tative binding of NO molecules on highly porous silica materials (> 1000 m2/g) is real-ized!
Next, the idea of combining NO effectivity and ROS production in one material is dis-cussed. In this context, exclusively Sortino et al. reported in 2009 the synthesis of chro-mophoric films that release both NO molecules and reactive oxygen species by light ex-citation.272 They nicely discussed the NO and ROS production efficiency separately re-garding visible light irradiation. But what they didn`t investigate was the effect when ROS meets NO species. From many biological reports it is known that NO molecules and superoxides react with each other resulting in highly antibacterial peroxynitrite ions (ONOO.-).273, 274 So far, there is no material known, releasing simultaneously NO mole-cules and superoxides species, induced by sunlight triggering, to defend bacterial pro-liferation on surfaces.
Fig. 9. Representative NO donors diazeniumdiolates (20), S-nitrosothiol (21), metal-nitrosyl complex (22) and nitrobenzene (23).