A Mini Review: Recent Advances in Surface Modification of Porous Silicon

materials

Review A Mini Review: Recent Advances in Surface Modiﬁcation of Porous Silicon

Seo Hyeon Lee 1 , Jae Seung Kang 2,3,* and Dokyoung Kim 1,4,5,6,*

1 Department of Biomedical Science, Graduate School, Kyung Hee University, Seoul 02447, Korea; [email protected] 2 Laboratory of Vitamin C and Anti-Oxidant Immunology, Department of Anatomy and Cell Biology, College of Medicine, Seoul National University, Seoul 03080, Korea 3 Institute of Allergy and Clinical Immunology, Medical Research Center, Seoul National University, Seoul 03080, Korea 4 Department of Anatomy and Neurobiology, College of Medicine, Kyung Hee University, Seoul 02447, Korea 5 Center for Converging Humanities, Kyung Hee University, Seoul 02447, Korea 6 Biomedical Science Institute, Kyung Hee University, Seoul 02447, Korea * Correspondence: [email protected] (J.S.K.), [email protected] (D.K.); Tel.: +82-02-961-0297 (D.K.)

Received: 26 November 2018; Accepted: 13 December 2018; Published: 15 December 2018

Abstract: Porous silicon has been utilized within a wide spectrum of industries, as well as being used in basic research for engineering and biomedical fields. Recently, surface modification methods have been constantly coming under the spotlight, mostly in regard to maximizing its purpose of use. Within this review, we will introduce porous silicon, the experimentation preparatory methods, the properties of the surface of porous silicon, and both more conventional as well as newly developed surface modification methods that have assisted in attempting to overcome the many drawbacks we see in the existing methods. The main aim of this review is to highlight and give useful insight into improving the properties of porous silicon, and create a focused description of the surface modification methods.

Keywords: surface modiﬁcation; porous silicon; silicon surface; carbonization; oxidation

1. Introduction Porous silicon (abbreviated as pSi) is a silicon formulation that has introduced nanopores in its microstructure. Porous silicon was discovered in the mid-1950s, and its unique physical, chemical, optical, and biological properties allowed us to develop new disciplines [1]. Porous silicon can be generated by electrochemical etching of crystalline silicon in hydrofluoric acid (HF) containing aqueous or non-aqueous electrolytes [2]. Silicon (Si) elements in Si wafer can be 2− dissolved out to a hexafluorosilane (SiF6 ) ion in the electrochemical etching stage, with each wafer generating a different pore diameter; p-type Si wafer (micropores, <2 nm), p+/p++/n+-type Si wafer (mesopores, 2–50 nm), and n+-type Si wafer (macropores, >50 nm) [1]. So far, various types of pSi materials have been reported, including pSi chip, pSi film, and pSi micro- and nano-particles (Figure1). The porosity, pore size, pore pattern, and particle size can be controlled by the fabrication parameters; HF concentration, current density, electrolyte composition, and wafer (dopant type, dopant density, crystallographic orientation) [3]. As compared to anodization and sonication processes, recently a new concept in electroless etching of Si powder has been reported that is an easily scalable process for the generation of pSi particles [4]. The pSi materials have been widely used in various industries and basic science. Due to a significant amount of research and the discovery of quantum confinement effects, photoluminescence,

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photoluminescence, and photonic crystal properties of pSi [5–9], the focus has mostly been on andcreating photonic optoelectronic crystal properties materials of [10,11], pSi [5 –displays[12,13],9], the focus has sensors mostly [14–16], been on and creating bio-imaging optoelectronic materials materials[17,18]. Recently, [10,11], displays the pSi micro- [12,13], and sensors nano-particles [14–16], and have bio-imaging been applied materials to drug [17 ,delivery18]. Recently, systems the and pSi micro-controlled and nano-particlesrelease systems, have by using been appliedthe biodegrada to drugtion delivery property systems of pSi and [1 controlled8–22]. One release such approach, systems, by“drug using loading the biodegradation in the pore and property surface of pSifunctionalization [18–22]. One such of pSi approach, materials “drug with loading disease in targeting the pore andmoiety”, surface was functionalization one of the biggest of pSi materialsleaps in the with fiel diseased of drug targeting delivery moiety”, systems. was one In ofaddition, the biggest the leapsnanostructured in the ﬁeld pSi of drug material delivery is a promising systems. In anode addition, material the nanostructuredfor high-performance pSi material lithium-ion is a promising batteries anode[23,24]. material With proper for high-performance surface modification, lithium-ion pSi mate batteriesrials have [23 shown,24]. With the suppression proper surface of pulverization, modiﬁcation, pSilow materials volume expansion, have shown and the a suppression long-term cycling of pulverization, stability in low the volumelithiation expansion, and delithiation and a long-term stages as cyclingnext-generation stability inlithium-ion the lithiation batteries. and delithiation stages as next-generation lithium-ion batteries.

Figure 1. Schematic illustration for the preparation of porous silicon by electrochemical etching and ultrasonication.Figure 1. Schematic A porous illustration silicon for layer the ispreparation generated of on porous the surface silicon of by the electrochemical silicon wafer using etching electro and chemicalultrasonication. etching, A and porous the layersilicon can layer be separatedis generated from on the wafersurface using of the lift-off silicon etch. wafer Porous using siliconelectro micro-chemical and etching, nano-particles and the can layer be prepared can be separated using ultrasonication from the wafer fracturing. using Thelift-off surface etch. of Porous the resulting silicon porousmicro- siliconand nano-particles is covered mainly can withbe prepared silicon-hydrogen using ultrasonication (Si–H) and partly fracturing. with silicon-oxygen The surface (Si–OH, of the Si–O–Si).resulting porous silicon is covered mainly with silicon-hydrogen (Si–H) and partly with silicon- oxygen (Si–OH, Si–O–Si). As we described above, the surface modification of pSi materials is imperative to improve the propertiesAs we of described pSi and itsabove, usage. the Essentially,surface modification freshly etched of pSi pSi materials has silicon is imperative hydrides to (Si–H) improve on thethe surfaceproperties and of residual pSi and oxides its usage. or fluorides Essentially, are removed freshly etched by the HFpSi electrolytehas silicon (Figurehydrides1). (Si–H) The reactive on the siliconsurface hydrides and residual on the oxides large or surface fluorides of pSi are is removed susceptible by tothe slow HF electrolyte oxidation in(Figure humid 1). air The [25 reactive,26]. In thesilicon field hydrides of optoelectronics on the large or surfac batterye of application, pSi is susceptible the oxidized to slow pSi oxidation could degrade in humid performance air [25,26]. ofIn materials.the field of However, optoelectronics the oxidized or battery pSi is applicatio necessaryn, in the the oxidized development pSi could of sensors, degrade photoluminescent performance of bio-imagingmaterials. However, materials, the and oxidized drug-delivery pSi is necessary systems. in Accordingly,the development surface of sensors, modification photoluminescent is the most importantbio-imaging component materials, in and terms drug-delivery of the use of pSisystems. materials. Accordingly, surface modification is the most importantIn this component review, we in have terms summarized of the use of the pSi conventional materials. surface modification methods, newly reportedIn this surface review, modification we have methods,summarized and ourthe perspectives.conventional surface modification methods, newly reported surface modification methods, and our perspectives. 2. Conventional Surface Modification Methods 2. ConventionalWe categorized Surface the well-known Modification and Methods typical surface modification methods into three categories: (i) hydrosilylationWe categorized & carbonization,the well-known (ii) an oxidation,d typical andsurface (iii) modification hydrolytic condensation. methods into three categories: (i) hydrosilylation & carbonization, (ii) oxidation, and (iii) hydrolytic condensation. 2.1. Hydrosilylation & Carbonization 2.1. HydrosilylationOver the last several& Carbonization decades, various attempts have been made at stabilizing the surface of porousOver silicon the last in order several to improvedecades, itsvarious suitability attempts for varioushave been applications made at stabilizing [27–31]. Amongthe surface them, of theporous silicon-carbon silicon in order (Si–C) to bondimprove formation its suitability yielded for avarious very stableapplications surface [27–31]. of pSi Among due to them, the low the electronegativitysilicon-carbon (Si–C) of carbon, bond which formation possesses yielded greater a kineticvery stable stability surface in comparison of pSi todue silicon-oxygen to the low (Si–O)electronegativity [32]. The most of carbon, ubiquitous which reaction possesses to make greater the Si–Ckinetic bond stability from in hydrogen-terminated comparison to silicon-oxygen pSi (Si–H) is(Si–O) hydrosilylation. [32]. The most Silicon ubiquitous hydride reaction (Si–H) moiety to make can the react Si–C with bond unsaturated from hydrogen-terminated carbon (alkene or alkyne)pSi (Si– andH) is bonds hydrosilylation. to one end of Silicon the hydrocarbon hydride (Si–H) reagent moie (Figurety can2 )[react33]. with unsaturated carbon (alkene or alkyne) and bonds to one end of the hydrocarbon reagent (Figure 2) [33].

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InIn thethe reportedreported methods,methods, thethe hydrosilylationhydrosilylation reactionreaction isis achievedachieved throughthrough thermalthermal (heat),(heat), photonphoton (light),(light), catalyst,catalyst, andand microwavemicrowave energyenergy (Figure(Figure2 a).2a). The The thermal thermal and and light-induced light-induced hydrosilylation hydrosilylation providesprovides a a means means to to place place a wide a wide variety variety of functional of functional groups groups on a pSion surfacea pSi surface that allow that further allow surfacefurther functionalization,surface functionalization, including including carboxylic carboxylic acid. The ﬁrstacid. hydrosilylation The first hydrosilylation methods were methods demonstrated were bydemonstrated Buriak and co-workersby Buriak and in early co-workers 1999, and in elaboratedearly 1999, byand Boukherroub, elaborated by Chazalviel, Boukherroub, Lockwood, Chazalviel, and manyLockwood, others and afterwards many others [1,32– afterwards35]. [1,32–35].

Figure 2. Surface chemistry for native porous silicon (pSi, Si–H). (a) Hydrosilylation with unsaturatedFigure 2. Surface carbon chemistry containing for reagents.native porous (b) Electrochemical silicon (pSi, Si–H). grafting (a) Hydrosilylation with halide containing with unsaturated reagents. (carbonc) Carbonization containing with reagents. acetylene. (b (d) )Electrochemical Water contact angle grafting of native with pSi andhalide its surfacecontaining chemistry reagents. product. (c) RCarbonization = simple aliphatic with acetylene. chain, functional (d) Water group contact containing angle of aliphatic native pSi chain. and its surface chemistry product. R = simple aliphatic chain, functional group containing aliphatic chain. At a similar time, Sailor and Salonen reported electrochemical reduction of organohalides and thermally-inducedAt a similar time, carbonization Sailor and approachesSalonen reported using el acetyleneectrochemical for the reduction surface graftingof organohalides of native and pSi surfacesthermally-induced (Si–H) (Figure carbonization2b,c) [ 36–38 approaches]. The electrochemical using acetylene reduction for the method surface allows grafting surface of native grafting pSi withsurfaces a methyl (Si–H) group, (Figure which 2b,c) is impossible[36–38]. The to electroc achievehemical through reduction typical thermally method induced allows surface hydrosilylation. grafting Thewith reaction a methyl of pSi group, with gas which phase is acetylene impossible generates to achieve many derivatives through oftypical Si–C bondsthermally on the induced surface ofhydrosilylation. pSi that is stable The in aqueousreaction media.of pSi with In the gas water phase contact acetylene angle measurement,generates many typical derivatives hydrosilylation of Si–C withbonds simple on the aliphatic surface chainsof pSi that and is an stable electrochemical in aqueous reactionmedia. In with the methylhalidewater contact gives angle a measurement, contact angle oftypical around hydrosilylation 100–120◦ and awith hydrophobic simple surface,aliphatic but chains the carbonization and an electrochemical reaction gives reaction lower contact with anglemethylhalide values at gives around a 53–80contact◦, whichangle mightof around vary depending 100–120° onand the a reactionhydrophobic temperature, surface, due but to thethe undesiredcarbonization Si–O reaction bond formation gives lower (Figure contact2d). angle values at around 53–80°, which might vary dependingThe important on the reaction points of temperature, hydrosilylation due and to the carbonization undesired Si–O reactions bond on formation the native (Figure silicon 2d). hydride terminalThe ofimportant the pSi surface points can of be hydrosilylation summarized as: and (i) maintainingcarbonization porous reactions structure, on the (ii) enhancingnative silicon the stabilityhydride ofterminal the pSi surface,of the pSi (iii) surface making can further be functionalizationsummarized as: possible,(i) maintaining and (iv) porous expanding structure, the scope (ii) ofenhancing application. the stability of the pSi surface, (iii) making further functionalization possible, and (iv) expanding the scope of application. 2.2. Oxidation 2.2. OxidationThe oxide-layer formation on the surface of pSi is also important to functionalize the materials. The hydrophobicThe oxide-layer property formation of nativeon the surface pSi became of pSi hydrophilic is also important after to the functionalize surface oxidation. the materials. This formulationThe hydrophobic can be property used for of bio-related native pSi applications became hydrophilic such as biosensors, after the surface drug delivery oxidation. systems, This andformulation photoluminescence can be used bio-imaging.for bio-related Generally, applications silicon-hydride such as biosensors, (Si–H) drug and delivery silicon-silicon systems, (Si–Si) and bondsphotoluminescence on the surface bio-imaging. of silicon materials Generally, can besilicon-hydride broken with the (Si–H) presence and ofsilicon-silicon an oxidant and (Si–Si) generates bonds theon the oxide surface layer; of hydrated silicon materials silicon oxide can (Si–OH)be broken and with silicon the oxidepresence (Si–O–Si) of an oxidant [39]. and generates the oxide layer; hydrated silicon oxide (Si–OH) and silicon oxide (Si–O–Si) [39].

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As an easy and facile method, gas-phase oxidants such as air (including oxygen) and ozone (O3) As an easy and facile method, gas-phase oxidants such as air (including oxygen) and ozone (O3) have been widely used for the oxidation of pSi (Figure 3a,b) [25,40]. At room temperature (25 °C), a have been widely used for the oxidation of pSi (Figure3a,b) [ 25,40]. At room temperature (25 ◦C), fairly thin oxide layer (Si–O–Si) grows over the course of several months in the presence of oxygen. a fairly thin oxide layer (Si–O–Si) grows over the course of several months in the presence of oxygen. At temperatures below 200 °C, pSi generates more hydroxy species silicon oxide layers on the surface. At temperatures below 200 ◦C, pSi generates more hydroxy species silicon oxide layers on the surface. Oxidation at 900 °C is sufficient to completely convert the pSi to silicon oxide, which has no Oxidation at 900 ◦C is sufﬁcient to completely convert the pSi to silicon oxide, which has no crystallinity crystallinity of silicon, although the transformation depends on the type of pSi. Interestingly, ozone of silicon, although the transformation depends on the type of pSi. Interestingly, ozone oxidation at oxidation at a room temperature of 25 °C gives a more hydrated silicon oxide than the thermal a room temperature of 25 ◦C gives a more hydrated silicon oxide than the thermal oxidation procedure oxidation procedure below 200 °C. below 200 ◦C.

Figure 3. Surface oxidation of native porous silicon (pSi, Si–H). (a) Oxidation with a gas-phase oxidant, Figure 3. Surface oxidation of native porous silicon (pSi, Si–H). (a) Oxidation with a gas-phase O2, in different temperatures. (b) Ozone (O3) oxidation. (c) Oxidation in aqueous media (H2O) with oxidant, O2, in different temperatures. (b) Ozone (O3) oxidation. (c) Oxidation in aqueous media (H2O) an oxidant (borate or nitrate). (d) Oxidation by organic oxidant (DMSO). (e) Water contact angle of with an oxidant (borate or nitrate). (d) Oxidation by organic oxidant (DMSO). (e) Water contact angle native pSi and its surface oxidized product. of native pSi and its surface oxidized product. Solution based oxidation of pSi is also widely used due to the facile operation and controllable oxidationSolution states. based In aqueousoxidation solutions, of pSi is also a silicon widely atom used on due the to pSi the surface facile operation can make and a 5-coordinate controllable oxidation states. In aqueous solutions, a silicon atom on the pSi surface can make a 5-coordinate intermediate with water molecules (H2O) and its cascade condensation, giving a Si–O–Si and hydroxylatedintermediate Si–OHwith water oxide molecules layer (Figure (H2O)3c) and [ 41 ].its The cascade oxidation condensation, rate and thicknessgiving a ofSi–O–Si the oxide and layerhydroxylated also can beSi–OH boosted oxide or layer regulated (Figure in the 3c) presence[41]. The ofoxidation an additional rate and oxidant, thickness such of as the borate oxide [42 layer] or nitratealso can [43 be]. boosted Similarly, or organic regulated solvents in the canpresence also be of used.an additional Dimethyl oxidant, sulfoxide such (DMSO) as borate can [42] also or oxidize nitrate the[43]. surface Similarly, of pSi organic in a mild solvents way andcan generatealso be used. the Si–O–Si Dimethyl layer sulfox onide the surface(DMSO) [ 44can]. Thealso Deuteriumoxidize the tracersurface study of pSi revealed in a mild that way DMSO and generate tended to the break Si–O–Si the Si–Si layer rather on the than surface Si–H [44]. bond The (Figure Deuterium3d). tracer studyIn revealed terms of that bonding DMSO energy, tended the to Si–Sibreak bond the Si–Si is weaker rather than than theSi–H Si–H bond bond, (Figure so the 3d). mild oxidant (DMSO,In terms pyridine, of bonding etc.) [45 ]energy, and thermal the Si–Si condition bond is (25 we◦C)aker tends than to the break Si–H the bond, Si–Si bond,so the andmild generate oxidant (DMSO, pyridine, etc.) [45] and thermal condition (25 °C) tends to break the Si–Si bond, and generate a Si–O–Si layer on the surface. Conversely, relatively reactive oxidants (O3, borate, nitrate, etc.) and harsha Si–O–Si conditions layer on (>100 the surface.◦C) tend Conversely, to break both relatively the Si–Si reactive and Si–H oxidants and generate (O3, borate, Si–O–Si nitrate, and etc.) Si–OH. and Theharsh generation conditions of (>100 an oxide °C) tend layer to on break the pSiboth surface the Si–Si can and be monitoredSi–H and generate by the waterSi–O–Si contact and Si–OH. angle measurement,The generation and of allan oxidationoxide layer products on the showpSi surface hydrophilic can be properties monitored below by the a 20 water◦ angle contact (Figure angle3e). measurement,The important and pointall oxidation to consider products about show the oxidation hydrophilic of the properties native pSi below surface a 20° can angle be summarized (Figure 3e). as: (i)The enhancing important hydrophilicity point to consider utilizing about its the bio-related oxidation of work, the native (ii) improving pSi surface functionalization can be summarized as muchas: (i) asenhancing possible, (iii)hydrophilicity enhancing quantumutilizing confinementits bio-related effects work, on (ii) the improving surface, and functionalization (iv) enhancing the as generationmuch as possible, of the reactive (iii) enhancing Si–O intermediate quantum confinemen to improvet modification.effects on the surface, and (iv) enhancing the generation of the reactive Si–O intermediate to improve modification. 2.3. Hydrolytic Condensation 2.3. Hydrolytic Condensation The oxidized pSi is a good platform for further surface modification. The hydrolytic condensation with organotrialkoxysilaneThe oxidized pSi is reagentsa good generatesplatform newfor Si–O–Sifurther bondssurface on modification. the surface in aThe protic hydrolytic solvent, condensation with organotrialkoxysilane reagents generates new Si–O–Si bonds on the surface in a

MaterialsMaterials 20182018,, 1111,, x 2557 FOR PEER REVIEW 5 5of of 14 14 protic solvent, such as ethanol at mild temperatures (25 °C) (Figure 4a) [46]. This simple silanol ◦ chemistrysuch as ethanol is well-known at mild and temperatures widely used (25 withinC) (Figurebio-related4a) works, [ 46]. Thissuch simpleas biomolecule silanol conjugation, chemistry is PEGylationwell-known (polyethylene and widely used glycol within attachment bio-related reaction), works, and such controlled as biomolecule degradation conjugation, of pSi materials PEGylation in bio-fluid(polyethylene [17,18,47–49]. glycol attachment In tests, there reaction), are two and standard controlled coupling degradation reagents; of pSi(i) 3-aminopropyltriethoxy- materials in bio-ﬂuid [17, silane18,47– (APTES,49]. In tests, X = thereNH2 arein Figure two standard 4b) [50,51], coupling and (ii) reagents; 3-mercaptopropyltriethoxy-silane (i) 3-aminopropyltriethoxy-silane (MPTES, (APTES, X = SHX = in NH Figure2 in Figure 4b). The4b) [hydrolytic50,51], and condensation (ii) 3-mercaptopropyltriethoxy-silane product with APTES gives (MPTES, primary X = amine SH in Figureterminals4b). onThe the hydrolytic surface condensationof pSi, and this product amine with moiety APTES can gives be chemically primary amine conjugated terminals using on theproteins, surface DNA, of pSi, antibodies,and this amine drugs, moiety and can many be chemically other molecules, conjugated via using amide proteins, coupling DNA, with antibodies, carboxylic drugs, acid and or many N- hydroxysuccinimideother molecules, via esters amide (Method coupling A) with [52,53]. carboxylic In a similar acid way, or N-hydroxysuccinimide the hydrolytic condensation esters (Methodproduct withA) [52 MPTES,53]. In gives a similar a thiol way, terminal the hydrolytic on the surface condensation of pSi, and product it can with be conjugated MPTES gives with a substrates thiol terminal via aon thiol-ene the surface reaction of pSi, (also and known it can beas conjugatedalkene hydrot withhiolation) substrates with via maleimide a thiol-ene (Method reaction B) (also [54]. known Both chemistriesas alkene hydrothiolation) show sufficiently with high maleimide yields of reaction (Method for B) most [54]. Bothof the chemistries oxidized pSi show materials. sufﬁciently Recently, high furtheryields of functionalization reaction for most of of oxidized the oxidized pSi using pSi materials. metal-free Recently, bioorthogonal further functionalizationclick chemistry was of oxidizedapplied forpSi the using peptide metal-free conjugation bioorthogonal [55,56]. click chemistry was applied for the peptide conjugation [55,56].

Figure 4. Surface modification of oxidized pSi. (a) Hydrolytic condensation with silanol reagents. Figure(b) Further 4. Surface functionalization modification of of hydrolytic oxidized condensationpSi. (a) Hydrolytic products condensation via amide with coupling silanol (Method reagents. A) and(b) Furtherthiol-ene functionalization addition (Method of hydrolytic B). A box incondensa Figure (tionb) indicates products the via surface amide functionalitycoupling (Method of hydrolytic A) and thiol-enecondensation addition with (Method APTES or B). MPTES A box onin theFigure oxidized (b) indicates pSi. the surface functionality of hydrolytic condensation with APTES or MPTES on the oxidized pSi. 3. Recently Developed Surface Modification Methods 3. RecentlyIn the Developed previous chapter, Surface we Modification summarized Methods the existing surface modification methods for pSi materials.In the Althoughprevious thechapter, known we methods summarized show superiorthe existing surface surface modification modification abilities, methods the practical for pSi materials.applications Although in industry the known and basic methods sciences show have superior met difficulties surface formodification many reasons. abilities, the practical applicationsAs an example,in industry the and thermally basic sciences induced have hydrosilylation met difficulties of for native many pSi reasons. typically required high ◦ heatAs (>200 an example,C), a very the specificthermally light induced with sufficientlyhydrosilylat highion of power, native an pSi air-sensitive typically required catalyst, high such heat as (>200rhodium °C), complexa very specific (known light as Wilkinson’swith sufficiently catalyst), high andpower, special an air-sensitive instruments. catalyst, This kind such of as experiment rhodium complexcould be (known conducted as Wilkinson’s by an expert catalyst), in an inert and atmospherespecial instruments. like a nitrogen This kind glovebox of experiment with home-built could be conductedinstruments. by an Occasionally, expert in an the inert resulting atmosphere pSi materiallike a nitrogen showed glovebox low efficiency with home-built of surface instruments. conversion Occasionally,and non-uniformed the resulting morphology. pSi material Surface modificationshowed low by efficiency oxidation of and surface hydrolytic conversion condensation and non- also uniformedcame with problems,morphology. such Surface as a loss modification of pSi during by modification, oxidation and long hydrolytic reaction condensation times (> 12 h), also undesired came withcross-linking problems, of such pSi, as and a loss low of efficiency pSi during because modifica oftion, generation long reaction of theby-product. times (> 12 h), To undesired overcome cross- these linkingissues, newof pSi, surface and low modification efficiency methodsbecause of have genera beention developed of the by-product. recently, such To asovercome (i) thermally these induced issues, newdehydrocoupling, surface modification (ii) ring-opening methods click have chemistry, been dev andeloped (iii) calcium-recently, orsuch magnesium-silicate as (i) thermally formation. induced dehydrocoupling,This chapter has summarized(ii) ring-opening the key click points chemistry, of these recentand (iii) advances calcium- in surfaceor magnesium-silicate modification of formation.pSi materials. This chapter has summarized the key points of these recent advances in surface modification of pSi materials.

3.1. Thermally Induced Dehydrocoupling

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TheMaterials hydrosilylation 2018, 11, x FOR PEER reaction REVIEW of the native pSi material, with alkene of alkyne moieties6 of 14on the organicThe reagent, hydrosilylation requires an reaction exclusion of the of oxidants native pSi including material, air, with moisture, alkene and of alkynewater in moieties order to on avoid the organic reagent, requires an exclusion of oxidants including air, moisture, and water in order to avoid productionThe of hydrosilylation an undesired silicoreactionn oxide. of the Annative alternative pSi material, method with isalkene dehydrogenative of alkyne moieties coupling on the with a production of an undesired silicon oxide. An alternative method is dehydrogenative coupling with trihydridosilaneorganic reagent, reagent requires (H an3Si-R, exclusion R = aliphatic, of oxidants aromatic) including (Method air, moisture, A, Figure and water 5a) in[57]. order However, to avoid this adehydrogenative trihydridosilaneproduction of ancoupling reagent undesired (Hmethod 3silicoSi-R,n Roxide. requires = aliphatic, An alternative transi aromatic)tion method metal (Method is dehydrogenativecatalysts A, Figure in order5 couplinga) [ 57to]. witheffect However, a the thistransformation, dehydrogenativetrihydridosilane and reagentthe coupling highly (H3 Si-R,reactive method R = aliphatic, catalysts requires aromatic) can transition lead (Methodto an metal oxidative A, Figure catalysts side 5a) reaction. [57]. in order However, Very to effect recently,this the dehydrogenative coupling method requires transition metal catalysts in order to effect the transformation,Kim and co-workers and the reported highly the reactive non-catalytic catalysts candehydrocoupling lead to an oxidative reaction side can reaction. proceed Very under recently, mild transformation, and the highly reactive catalysts can lead to an oxidative side reaction. Very recently, Kimthermal and conditions co-workers on reported the pSi the surface non-catalytic (Method dehydrocoupling B, Figure 5b) [58,59]. reaction The can first proceed investigation under mildwas thermalKim conditionsand co-workers on thereported pSi surface the non-catalytic (Method dehydrocoupling B, Figure5b) [ 58reaction,59]. Thecan proceed first investigation under mild was successfully carried out with the pSi film and an octadecysilane, H3Si(CH2)17CH3 (abbreviated as H3Si- successfullythermal carriedconditions out on with the thepSi surface pSi film (Method and an B, octadecysilane, Figure 5b) [58,59]. H Si(CHThe first) investigationCH (abbreviated was as 18 3 2 17 3 C ). Thesuccessfully pSi film carried was heated out with at the 80 pS°Ci forfilm 1–24 and◦ anhours, octadecysilane, in the presence H3Si(CH of2 )a17 neatCH3 (abbreviated silane reagent. as H Infrared3Si- H3Si-C18). The pSi film was heated at 80 C for 1–24 hours, in the presence of a neat silane reagent. spectrumC18). The analysis pSi film of wasnative heated the pSiat 80 film °C for and 1–24 reaction hours, productin the presence (pSi-Si(C of a 18neat)) displayed silane reagent. bands Infrared associated Infrared spectrum analysis of native the pSi film and reaction product (pSi-Si(C18)) displayed−1 bands with spectrumpSi-H and analysis an organosilane of native the reagent pSi film (Figure and reaction 6a). A product strong (pSi-Si(C Si lattice18)) mode displayed at 515 bands cm associated was observed −1 associated with pSi-H and an organosilane reagent (Figure6a). A strong Si lattice− mode1 at 515 cm in thewith Raman pSi-H spectrum and an organosilane of pSi-H and reagent pSi-Si(C (Figure18), 6a). which A strong indicated Si lattice a moderetained at 515 crystallinity cm was observed of pSi after wasthe reaction observedin the Raman (Figure in the spectrum Raman 6b). The of spectrum pSi-H preservation and of pSi-Si(C pSi-H of and18 ),an which pSi-Si(Copen indicated pore18), whichstructure a retained indicated was crystallinity observed a retained of pSiby crystallinity afterscanning ofelectron pSithe after microscopereaction the reaction(Figure (SEM) 6b). (Figure images The 6preservationb). (Figure The preservation 6c). of Th ane superhydrophobicopenof pore an openstructure pore surfacewas structure observed property was by was observedscanning observed by electron microscope (SEM) images (Figure 6c). The superhydrophobic surface property was observed scanningin the water electron contact microscope angle measurement (SEM) images (>150°) (Figure after6 c).rinsing The superhydrophobicwith HF (Figure 6d), surface and it property suggests was that in the water contact angle measurement (>150°) after rinsing◦ with HF (Figure 6d), and it suggests that observedthis reaction inthe occurred water contactbetween angle the Si measurement element on (>150the pSi) afterand rinsingthe Si element with HF on (Figure the reagent,6d), and not it suggeststhis thatreaction this reactionoccurred occurredbetween the between Si element the Si on element the pSi on and the the pSi Si and element the Si on element the reagent, on the not reagent, throughthrough the oxidizedthe oxidized pSi pSi surface. surface. not through the oxidized pSi surface.

Figure 5. Thermally induced dehydrocoupling of the native pSi surface (Si–H) with trihydridosilane Figure 5. Thermally induced dehydrocoupling of the native pSi surface (Si–H) with trihydridosilane reagents (H3Si-CH2-R, marked in blue). (a) Dehydrocoupling by catalyst or light (Method A) and mild reagentsFigureheat 5. (HThermally(Method3Si-CH B).2 -R,induced Inset marked figure: dehydrocoupling in water blue). contact (a) Dehydrocoupling angleof the of nati reactionve pSi product.bysurface catalyst (Si–H)(b) orSummary lightwith (Methodtrihydridosilane of reaction A) and mildreagentsproducts heat (H (Method3Si-CH of pSi2 -R,with B). marked Inset various ﬁgure: in trihydridosilane blue). water (a) contact Dehydrocoupling reagents. angle of reaction by catalyst product. or light (b) Summary(Method A) of and reaction mild productsheat (Method of pSi B). with Inset various figure: trihydridosilane water contact reagents. angle of reaction product. (b) Summary of reaction products of pSi with various trihydridosilane reagents.

Figure 6. Cont.

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Figure 6. CharacterizationCharacterization of of a a thermally induced induced dehy dehydrocouplingdrocoupling reaction reaction product product of of pSi with

octadecysilane (H 3Si-CSi-C1818).). (a ()a )Attenuated Attenuated total total reflectance reﬂectance Fourier-transform Fourier-transform infrared infrared (ATR-FTIR) (ATR-FTIR) spectra of of the the reagent reagent (H (H3Si-C3Si-C1818) and) and products products (pSi-Si-C (pSi-Si-C18).18 (b).) Raman (b) Raman spectra spectra obtained obtained before before (As- (As-etchedetched pSi) pSi)and after and afterreaction reaction (HF rinsed (HF rinsed pSi-Si-C pSi-Si-C18). (c) 18Scanning). (c) Scanning electron electron microscope microscope (SEM) images (SEM) ofimages pSi before of pSi (As-etched before (As-etched pSi) and pSi) after and reaction after reaction (HF rinsed (HF pSi-Si-C rinsed pSi-Si-C18) (plan18 view).) (plan (d view).) Water (d contact) Water anglecontact image angle and image illustration and illustration of the of surface the surface after after reaction. reaction. Reproduced Reproduced with with permission permission from from [58]. [58]. Copyright 2016 Wiley-VCH.

The thermally induced dehydrocoupling reacti reactionon with a trihydridosilane reagent shows remarkable tolerance tolerance to to oxidants oxidants such such as as air air and and water. water. The The reaction reaction of of pSi with a mixture of water and octadecysilane in open air conditions shows an adverse effect, and a grafting reaction. They also followed the effect effect of of surface surface modification modification on on the the intrinsic intrinsic photoluminescence photoluminescence quantum-confined quantum-confined pSi material. For For this this study, study, n-type pSi silicon film film was prepared by electrochemical etching under UV light irradiation. Interestingly, Interestingly, the the preserved preserved ph photoluminescenceotoluminescence was was monitored monitored after after the the reaction, reaction, and the product maintained their emissions for a long time (> 9 9 days) days) within within the the aerated aerated water whilst immersed. TheyThey also also demonstrated demonstrated dehydrocoupling dehydrocoupling reactions reactions with variouswith various trihydridosilane trihydridosilane reagents reagentsand tested and their tested suitability their suitability for further fororganic further functionalizationorganic functionalization reaction. reaction. Many functional Many functional groups groupssuch as such bromide, as bromide, azide, primary azide, amine,primary propargyl, amine, propargyl, and perfluorocarbon and perfluorocarbon were able towere be introducedable to be introducedonto the surface onto ofthe the surface resulting ofproductions. the resulting Further productions. conjugation Further chemistry conjugation and electrostatic chemistry drugand electrostaticloading demonstrations drug loading were demonstrations successfully carriedwere successfully out with a primarycarried out amine-terminal with a primary production amine- terminaltoward fluorescein production isothiocyanate toward fluorescein and negatively isothiocyanate charged and drugs negatively (ciprofloxacin charged in drugs this study).(ciprofloxacin in this study). 3.2. Ring-opening Click Chemistry 3.2. Ring-openingDriven by the Click desire Chemistry for functionalization and stabilization of the pSi surface, the reactive Si-H surfaceDriven often by converted the desire to for Si–O functionalization moiety. As we described and stabilization in the previous of the pSi chapter, surface, oxidized the reactive pSi surfaces Si-H surfacecan be functionalized often converted by theto Si–O grafting moiety. of organotrialkoxysilanes As we described in via the a hydrolyticprevious chapter, condensation oxidized reaction. pSi surfacesDespite theircan be utility, functionalized the critical by limitations, the grafting such of or asganotrialkoxysilanes undesired cross-linking, via a which hydrolytic results condensation in an overly reaction.thick coating Despite or clogging their utility, of the the pore critical of pSi, limitations, long reaction such times, as undesired and low coupling cross-linking, efficiency which give results many indrawbacks an overly to thick the practical coating applicationsor clogging ofofpSi the [ 1po,60re]. of pSi, long reaction times, and low coupling efficiencyIn 2016, give Kim many and drawbacks co-workers to the developed practical a applications facile surface of modification pSi [1,60]. method of oxidized pSi via ring-openingIn 2016, Kim clickand co-workers chemistry, using developed 5-membered a facile heterocyclicsurface modification compounds method containing of oxidized Si–S orpSi Si–N via ring-openingbonds within theclick ring chemistry, (Figure7 )[using61]. 5-membered heterocyclic compounds containing Si–S or Si–N bonds within the ring (Figure 7) [61].

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Figure 7. Ring-opening click chemistry of oxidized pSi surface (Si–OH) with cyclic-silane reagent. Figure 7. Ring-opening click chemistry of oxidized pSi surface (Si–OH) with cyclic-silane reagent. (a) (a) Reaction mechanism. DCM: dicholoromethane. R1 = OMe, Me. R2 = H, Me. X = S, N. (Reactionb) Cyclic-silane mechanism. reagents. DCM: (c dicholoromethane.,d) Further functionalization R1 = OMe, ofMe. ring-opening R2 = H, Me. X click = S, chemistryN. (b) Cyclic-silane products (pinkreagents. and (c olive),d) Further via amide functionalization coupling with of ring-opening succinic anhydride click chemistry (purple) products and tandem (pink coupling and olive) with via epoxyamide containingcoupling with reagent succinic (red). anhydride DMTSCP: (purple) 2,2-dimethoxy-1-thia-2-silacyclopentane. and tandem coupling with epoxy BADMSCP:containing Nreagent-n-butyl-aza-2,2-dimethoxy-silacyclopentane. (red). DMTSCP: 2,2-dimethoxy-1-thia-2-silacyclopentane. DMDASCP: 2,2-dimethoxy-1,6-diaza-2-silacyclooctane. BADMSCP: N-n-butyl-aza-2,2- MATMSCP:dimethoxy-silacyclopentane. N-methyl-aza-2,2,4-trimethyl-silacyclopentane. DMDASCP: 2,2-dimethoxy-1,6-diaza-2-silacyclooctane. MATMSCP: N- methyl-aza-2,2,4-trimethyl-silacyclopentane. The name of the reaction originated from simple click chemistry that showed a lack of by-product withThe a high name yield of and the widereaction scope originated of applicability. from simple The hydroxy click chemistry moiety onthat the showed oxidized a pSilack surface of by- attackedproduct with the silicon a high element yield and on wide the cyclic-silane scope of applicability. reagent and The the hydroxy Si–X (X =moiety S or N) on bond the oxidized was broken pSi (ring-opening)surface attacked with the asilicon proton element transfer on (H-transfer) the cyclic-silane within reagent a mild and condition the Si–X (Figure (X = S 7ora). N) Originally, bond was thisbroken kind (ring-opening) of click chemistry with was a proton proposed transfer by Arkles (H-transfer) and co-workers within ina Gelestmild condition Inc. USA (Figure in 2015 [7a).62]. TheyOriginally, found this an kind unusually of click fast chemistry and efficient was proposed surface graftingby Arkles ability and co-workers of the cyclic in Gelest azasilane Inc. reagentUSA in (Si–N2015 [62]. bond They in the found ring). an With unusually promising fast and results efficient based surface on the grafting amorphous ability silica of materials,the cyclic azasilane Kim and co-workersreagent (Si–N expanded bond in the the ring). research With field promising to oxidized results pSi based films on andthe amorphous pSi micro- silica and nano-particlesmaterials, Kim usingand co-workers other heterocyclic expanded silanes the research (Figure 7fieldb). Theto oxid thermogravimetricized pSi films and analysis pSi micro- (TGA) and of nano-particles the resulting production,using other preparedheterocyclic through silanes hydrolytic (Figure 7b). condensation The thermogravimetric and ring-opening analysis click chemistry,(TGA) of the gave resulting ~2–4% andproduction, ~8% mass prepared changes, through representing hydrolytic a high condensati efficiency foron thisand newring-opening type of method click chemistry, for surface gave grafting. ~2– The4% and maintenance ~8% mass of changes, the crystallinity representing of the a siliconhigh efficiency skeleton wasfor this then new verified type usingof method a powder for surface X-ray diffractiongrafting. The (XRD) maintenance measurement. of the crystallinity of the silicon skeleton was then verified using a powder X-rayWithin diffraction these (XRD) promising measurement. surface grafting results, a one-pot tandem synthesis was demonstrated, usingWithin an amine-containing these promising cyclicsurface azasilane grafting reagentresults, a (Figure one-pot7c,d) tandem [ 61, 63synthesis]. The was ring-opening demonstrated, click reactionusing an generated amine-containing a primary cyclic amine azasilane at the surface, reagent which (Figure then 7c,d) reacted [61,63]. to the The succinic ring-opening anhydride click or epoxyreaction containing generated reagent. a primary Through amine these at the tandem surface, coupling which then reactions, reacted new to functionalthe succinic groups, anhydride such asor carboxylicepoxy containing acid (–COOH) reagent. orThrough hydroxyl these (–OH) tandem moiety coupling were generated, reactions, new and, func therefore,tional showedgroups, such a good as casecarboxylic for application acid (–COOH) using bio-conjugation.or hydroxyl (–OH) moiety were generated, and, therefore, showed a good case Infor thisapplication study, the using authors bio-conjugation. also tested the compatibility of the surface functionalization with proteinIn loadedthis study, porous the siliconauthors nano-particles also tested the (pSiNPs) compatibility (Figure of8). the The surface nano-particle functionalization formulation with of pSiprotein maintained loaded itsporous pore structure,silicon nano-particles its protective (pSi sensitiveNPs) (Figure payloads 8). from The denaturingnano-particlein vitroformulationor in vivo of, andpSi maintained has proven its to pore be a structure, promising its delivery protective vessel. sensitive A model payloads protein from lysozyme denaturing was in loadedvitro or intoin vivo the, and has proven to be a promising delivery vessel. A model protein lysozyme was loaded into the pSiNPs, and then the surface of resulting pSiNP was functionalized with thia-silane reagent in either

Materials 2018, 11, 2557 9 of 14

Materials 2018, 11, x FOR PEER REVIEW 9 of 14 pSiNPs, and then the surface of resulting pSiNP was functionalized with thia-silane reagent in either DCM oror nn-Hex-Hex (Figure(Figure8 a).8a). The The generation generation of of the the surface surface thiol thiol species species was was within within 72 h72 and h and retained retained its activity;its activity; 98% 98% (reaction (reaction in DCM) in DCM) and 72%and (reaction72% (reaction in n-Hex) in n-Hex) (Figure (Figure8b). By 8b). contrast, By contrast, lysozyme lysozyme loaded pSiNPsloaded pSiNPs with the with hydrolytic the hydrolytic condensation condensation reaction usingreaction MPTES using gave MPTES only gave 68% only activity 68% of activity lysozyme of afterlysozyme release. after release.

Figure 8. Application of ring-opening click chemistry for substrate loaded pSi nano-particles (pSiNPs). (Figurea) procedure 8. Application used to loadof ring-opening lysozyme into click pSiNPs chemistr andy for modify substrate the resulting loaded pSi particles nano-particles with the cyclic-silane(pSiNPs). (a) reagent procedure (thia-silane used to in load this lysozyme study). DCM: into dichloromethane.pSiNPs and modify (b) the Release resulting profile particles of lysozyme with fromthe cyclic-silane unmodified pSiNPsreagent and(thia-silane thia-silane in functionalizedthis study). DCM: pSiNPs dichloromethane. that were prepared (b) in Release different profile solvents, of DCMlysozyme or n -Hex.from Reproducedunmodified withpSiNPs permission and thia-silane from [61 functionalized]. Copyright 2016 pSiNPs American that Chemicalwere prepared Society. in different solvents, DCM or n-Hex. Reproduced with permission from [61]. Copyright 2016 American 3.3. Calcium- or Magnesium-Silicate Formation Chemical Society. A formulation of a pSi core with an oxide layer shell is an attractive platform for further chemical conjugation,3.3. Calcium- offeringor Magnesium-Silicate increased stability, Formation enhanced photoluminescence intensity, and a controlled drug delivery release system. In some cases, the oxide layer formation of pSi has been used to trap the A formulation of a pSi core with an oxide layer shell is an attractive platform for further chemical substrates within the nanostructure [43,49,64]. In 2016, Kang and co-workers reported a facile pSi conjugation, offering increased stability, enhanced photoluminescence intensity, and a controlled surface oxidation method through the formation of a calcium silicate (Ca-silicate) insoluble shell drug delivery release system. In some cases, the oxide layer formation of pSi has been used to trap (Figure9)[ 48]. It was shown in this study that the high concentration calcium (II) ion can react the substrates within the nanostructure [43,49,64]. In 2016, Kang and co-workers reported a facile pSi with a hydroxylated silicon surface and hydrolyzed orthosilicic acid (Si(OH) ) and form the Ca SiO surface oxidation method through the formation of a calcium silicate (Ca-silicate)4 insoluble2 shell4 precipitation (calcium silicate shell) on the surface, trapping the substrate, siRNA. The calcium silicate (Figure 9) [48]. It was shown in this study that the high concentration calcium (II) ion can react with shell could also be degraded in the aqueous media and release the substrate, but the release rate was a hydroxylated silicon surface and hydrolyzed orthosilicic acid (Si(OH)4) and form the Ca2SiO4 slowerprecipitation than the(calcium known silicate physical shell) trapping on the method. surface, In trapping addition, the the substrate, oxide shell siRNA. formation The calcium process dramaticallysilicate shell could increased also the be quantumdegraded yield in the of aqueou pSiNPss frommedia 0.1% and to release 21%. The the calciumsubstrate, silicate but the formation release withrate was pore slower sealing than was verifiedthe known by transmissionphysical trapping electron method. microscope In addition, (TEM) images,the oxide energy shell dispersiveformation X-rayprocess (EDC) dramatically analysis, Fourierincreased transform the quantum infrared yield (FTIR) of pSiNPs spectrum, from and 0.1% nitrogen to 21%. adsorption–desorption The calcium silicate isothermformation analysis. with pore sealing was verified by transmission electron microscope (TEM) images, energy dispersiveAfter basicX-ray characterizations, (EDC) analysis, the Fourier authors transf wentorm on to infrared demonstrate (FTIR) a potential spectrum, use forand the nitrogen disease specificadsorption–desorption gene delivery isotherm approach, analysis. by using a modification of the calcium silicate coated pSiNPs (Ca-pSiNPs)After basic surface. characterizations, First, the surface the authors of the siRNA went on loaded to demonstrate Ca-pSiNPs a (Ca-pSiNP-siRNA, potential use for the ~20 disease wt % siRNA)specific gene was functionalizeddelivery approach, with by 3-aminopropyl-dimethylethoxysilane using a modification of the calcium silicate (APDMES) coated via pSiNPs hydrolytic (Ca- condensation.pSiNPs) surface. The First, Ca-pSiNP-siRNA the surface of wasthe thensiRNA conjugated loaded Ca-pSi with aNPs bi-functional (Ca-pSiNP-si polyethyleneglycolRNA, ~20 wt % (PEG),siRNA) maleimide-PEG-succinimidyl was functionalized with 3-aminopropyl-dimethylethoxysilane carboxy methyl ester (MAL-PEG-SCM), (APDMES) via amide via hydrolytic coupling betweencondensation. amine The and Ca-pSiNP-siRNA succinimidyl carboxy was then methyl conj ester.ugated The with remaining a bi-functional distal end polyethyleneglycol of the PEG was additionally(PEG), maleimide-PEG-succinimidyl conjugated, targeting RVG carboxy (rabies meth virusyl ester glycoprotein) (MAL-PEG-SCM), and cell penetrating via amide coupling peptides between amine and succinimidyl carboxy methyl ester. The remaining distal end of the PEG was additionally conjugated, targeting RVG (rabies virus glycoprotein) and cell penetrating peptides (mTP; myristolated transportan). A mice study was conducted using mice with brain injuries, and it

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Materials 2018, 11, x FOR PEER REVIEW 10 of 14 (mTP; myristolated transportan). A mice study was conducted using mice with brain injuries, and it concludedconcluded thatthat thethe formulationformulation waswas successfullysuccessfully delivereddelivered toto thethe injuredinjured sitesite andand releasedreleased thethe activeactive siRNAsiRNA payload.payload. AsAs aa follow-upfollow-up study,study, WangWang and and co-workers co-workers reportedreportedthat that aa similarsimilar demonstrationdemonstration basedbased onon thethe magnesiummagnesium silicatesilicate (Mg-silicate)(Mg-silicate) trappingtrapping ofof bovinebovine serumserum albuminalbumin (BSA)(BSA) intointo thethe pSipSi micro-particlesmicro-particles withwith theirtheir photoluminescencephotoluminescence intensityintensity analysisanalysis [[20].20]. TheyThey foundfound thatthat thethe Mg-silicateMg-silicate cancan suppresssuppress burstburst releasesreleases ofof thethe payload,payload, andand thisthis kinetickinetic releaserelease cancan bebe trackedtracked usingusing thethe photoluminescencephotoluminescence ofof pSipSi micro-particles.micro-particles.

Figure 9. Schematic illustration of the calcium (Ca2+)- or magnesium (Mg2+)-silicate formation of Figure 9. Schematic illustration of the calcium (Ca2+)- or magnesium (Mg2+)-silicate formation of pSiNPs surface. A thin oxide layer and orthosilicic acid (Si(OH)4) is generated from the pSiNPs in aqueouspSiNPs surface. media and A thin forms oxide a precipitate layer and that orthosilicic traps the substrateacid (Si(OH) (olive)4) is withingenerated the nanostructure.from the pSiNPs in aqueous media and forms a precipitate that traps the substrate (olive) within the nanostructure. In this chapter, we cover more recently developed surface modification methods for pSi materials with theirIn this detailed chapter, mechanisms, we cover key more benefits, recently and applications.developed surface We also modification summarize the methods key advantages for pSi andmaterials disadvantages with their compared detailed mechanisms, to existing surface key benefits, modification and applications. methods (Table We 1also). summarize the key advantages and disadvantages compared to existing surface modification methods (Table 1). Table 1. Summary of advantages, disadvantages, and applications for each surface modification methodsTable 1. ofSummary pSi materials. of advantages, disadvantages, and applications for each surface modification methods of pSi materials. Methods Advantages Disadvantages Methods Advantages Disadvantages - Single-step - Requires harsh reaction condition Carbonization - Single-step - Requires harsh reaction condition Carbonization - Enhance hydrophobicity - Requires special instruments (Si–H to Si–C) - Enhance hydrophobicity - Requires special instruments (Si–H to Si–C) - Enhance stability - Need practiced hands - Enhance- Facile stability method - Need- Loss practiced of pSi during hands modification Oxidation - Facile- Enhance method hydrophilicity - Loss- Difficult of pSi to during control modification oxidation state (Si–HOxidation to Si–OH, Si–O–Si) - Enhance- Bio friendly hydrophilicity - Difficult- Undesired to control pore clogging oxidation state (Si–H to Si–OH, Si–O–Si) - Facile method - Loss of pSi during modification Hydrolytic condensation - Bio friendly - Undesired pore clogging - Facile- Functional method group diversification - Loss- Time of consumingpSi during modification process (Si–OHHydrolytic to Si–O–Si–R) condensation - Functional- Bio friendly group diversification - Time- Undesired consuming cross-linking process of pSi (Si–OH to Si–O–Si–R) - Single-step, mild condition Thermal dehydrocoupling - Bio friendly - Undesired- Undesired cross-linking Si-O formation of pSi - Enhanced hydrophobicity (Si–H to Si–Si–R) - Single-step, mild condition - Large amount of reagent (neat) Thermal dehydrocoupling - Functional group diversification - Undesired Si-O formation - Enhanced hydrophobicity (Si–H to Si–Si–R) - High yield - Large amount of reagent (neat) Ring opening click chemistry - Functional- Single-step, group facile diversification method - Reaction only in aprotic solvent (Si–OH to Si–O–Si–R) - High- Functional yield group diversification - Only for hydroxylated pSi Ring opening click chemistry - Single-step,- Inert to payloadfacile method - Reaction only in aprotic solvent (Si–OH to Si–O–Si–R) - Functional- Single-step, group facile diversification method - Only for hydroxylated pSi Ca-/Mg-silicate formation - Inert- High to payload loading yield - Exothermic reaction (Si–OH to Si–O–Ca/Mg–O–Si) - Single-step,- Photoluminescence facile method generation - Require further modification Ca-/Mg-silicate formation - Further surface modification - High loading yield - Exothermic reaction (Si–OH to Si–O–Ca/Mg–O– - Photoluminescence generation - Require further modification Si) 4. Summary and Outlook - Further surface modification Surface modification can provide physical- or chemical-stability, functionality, and chemically 4. Summary and Outlook reactive sites, which can be used to interact or conjugate substrates. For this reason, there has been a desperateSurface need modification for the development can provide of aphysical- more advanced or chem surfaceical-stability, modification functionality, method ofand the chemically materials. Thereactive existing sites, methods which can have be beenused confrontedto interact or by co manynjugate challenges, substrates. such For as this hydrosilylation, reason, there oxidation,has been a desperate need for the development of a more advanced surface modification method of the materials. The existing methods have been confronted by many challenges, such as hydrosilylation, oxidation, and hydrolytic condensation. In this focused review, we introduced three recently

Materials 2018, 11, 2557 11 of 14 and hydrolytic condensation. In this focused review, we introduced three recently developed and differing surface modification methods of porous silicon; hydrolytic condensation, ring-opening click chemistry, and calcium- or magnesium-silicate formation. These new methods have shown remarkable advantages compared to more traditional existing methods, in an attempt to develop this research to improve the properties of porous silicon. As scientists undertaking research within the field of material science, especially based on porous silicon, we believe that the next-generation of surface modification methods and reagents for porous silicon will have several focuses: (i) ease-of-use, (ii) high reproducibility, (iii) high bio-applicability (low toxicity), (iv) controllable surface modification ratio, (v) minimizing or maximizing the pore collapse during the modification, and (vi) multi-functional surface modified moiety. Advances in surface modification methods and reagents can expand their application throughout many different fields. In particular, those that are currently attracting attention, such as polymer fabrication, food preservation coating, cell growth regulation, tissue engineering, and antifungal coating for medical devices. In addition, this new fabrication technique is expected to be used in certain applications (i.e., water-repellent coating, photolithography, etc.). We hope this review offers some basic information about porous silicon and surface modification methods, and that it can inspire other scientists to develop more advanced methods in order to improve these materials.

Author Contributions: S.H.L., J.S.K., and D.K. designed the content and wrote this review. Funding: This research was supported by the Bio & Medical Technology Development Program of the National Research Foundation (NRF) of Korea funded by the Ministry of Science & ICT (NRF-2018-M3A9H3021707). D. Kim gives thanks to the financial support from the Basic Science Research Program through the National Research Foundation (NRF) of Korea funded by the ministry of Education (NRF-2018-R1A6A1A03025124, NRF-2018-R1D1A1B07043383). J.S. Kang gives thanks to the financial support from the Basic Science Research Program through the National Research Foundation (NRF) of Korea funded by the Ministry of Education (2017R1A2B2010948). Acknowledgments: Thanks to Neil P. George and Sujin Jung for the linguistic editing. Conflicts of Interest: The authors declare no conflict of interest.

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