Porous silicon degradation

Scheme 1 Proposed mechanism for porous silicon degradation in aqueous solutions, adapted from Allongue et al. (1993). (a) A Si-H-terminated surface immersed in H2O. (b) The Si-H bond undergoes hydrolytic attack and is converted to Si-OH and produces a hydrogen molecule, (c) The Si-OH at the surface polarizes and weakens the Si-Si backbonds, which are then attacked by H2O, producing HSi(OH)3. (d) In solution, the HSi(OH)3 molecule is quickly converted to Si(OH)4 releasing a second hydrogen molecule...

Matrix-free direct laser desorption ionization of analyte has been studied on different kinds of surfaces without real success because degradation of the sample is usually observed. However, good results were obtained with the method called surface-activated laser desorption ionization (SALDI) [43] which uses graphite as the surface. But the use of porous silicon as a new surface is more promising and has led to the development of a new method called desorption ionization on silicon (DIOS) [44], Unlike the other matrix-free laser desorption ionization methods, DIOS allows ion formation from analyte with little or no degradation. 

Fe-passivated porous silicon (PS) with non-degrading photoluminescence (PL) was prepared by treating P/P+-type, boron-doped single crystal (111) Si wafers at 140 °C in an aqueous solution with 40 wt% HF and 0.3 M Fe(N03)3 (volume ratio, 7/6) [8, 9j. As shown in Figure 7.2, for freshly prepared samples, the PL peak intensity is 2-2.5 times stronger than that of ordinary PS. Furthermore, unlike... 

Ongoing investigations into the chemistry of porous silicon surfaces seek to develop methods for the preparation of chemically functional interfaces that protect the underlying silicon nanocrystallites from degradation without changing or annihilating their intrinsic behavior. The native, hydride-terminated surface is only metastable under ambient conditions and oxidation of freshly prepared porous silicon commences within minutes when exposed to air. While surface oxide can suitably passivate the nanocrystalline silicon and stabilize its photoluminescence, the electrically insulating and structurally defective character of this oxide layer... 

With the aim of developing a drug delivery vehicle, hydrosilylated and thermally oxidized porous silicon microparticles were injected into the vitreous of rabbit eyes (Cheng et al. 2008). It was noted that the particles settled into the inferior vitreous cavity over a few days. Degradation of the hydrosilylated particles took considerable time (>4 months) in comparison to untreated particles which degraded within a period of 3 weeks. No adverse effects were observed in ocular tissues including the retina and the lens. Furthermore, normal ocular pressure was maintained. 

In vitro degradation studies of porous silicon have shown release of orthosilicic acid from both anodized films and microparticles using molybdate blue assays or ICP analysis (Anderson... 

Administration route 1 Bioactive 1 Biocompatibility 1 Biodegradable porous silicon 1 Bioresorbable property 1 Composite structures 5 Degradation products 5 Drug delivery 1 In vitro drug 2 In vivo drug 2 Intravenous delivery 4 Intravitreal delivery 4 Microparticles 1 Nanoparticles 1 Oral delivery 2 Peptide 3... 

Astrova EV, Emisev VV, Lebedev AA, Poloskin DI, Remenyuk AD, Rud YV, Khartsiev VE (1995) Degradation of the photoluminescence of porous silicon caused by Co y radiation. Semiconductors 29 674-676... 

Choi S-H, Chung H, Shin G-S (1995) Conditions of luminescence degradation or enhancement in porous silicon. Solid State Commun 95(6) 341-345... 

Du XW, Jin Y, Zhao NQ, Fu YS, Kulinich SA (2008) Controlling surface states and photoluminescence of porous silicon by low-energy-ion irradiation. Appl Surf Sci 254(8) 2479-2482 El Houichet H, Oueslati M, Bessais B, Ezzaouia H (1997) Photoluminescence enhancement and degradation in porous silicon evidence for nonconventional photoinduced defects. J Lumin... 

As with solution loading, the surface passivation of the porous silicon is important when loading by melt intrusion - if the active is hydrophilic, for example, then the payload is likely to be small for hydride-passivated porous silieon. Some actives can, during loading, chemically oxidize or etch the porous silicon at elevated temperature, while some actives can degrade when in contact with the internally passivated silicon hydride surface. For the majority of applications, the highly reactive hydride surface will be modified before loading, more often than not by thermal oxidation (with, of course, subsequent reductions in pore volume and payload). 

The next topic addressed here is the current prime food manufacturer motivation for exploring the use of mesoporous silicon in food protection of established nutrients. This remains a prevailing issue for functional foods, irrespective of whether or not the degradation product of porous silicon becomes accepted as an essential nutrient in its own right. 

Fig. 4 UV Absorption scans of free retinol and retinol entrapped within mesoporous sihcon (Skaria, 2009, Photoprotection of retinol in porous silicon, Intrinsiq Materials Ltd report, unpublished ). ( ) Undegraded, (B) free retinol degraded by long-wave UV light, (Q retinol-loaded pSi membrane exposed to UV light, (D) retinol-loaded pSi membrane covered with aluminum foil and exposed to UV light...

Mclnnes S, Voelcker N (2012) Porous silicon-based nanostructured microparticles as degradable supports for solid-phase synthesis and release of oligonucleotides. Nanoscale Res Lett 7(1) 1-10 Mclnnes SJP, Szili EJ, Al-Bataineh SA, Xu J, Alf ME, Gleason KK, Short RD, Voelcker NH (2012) Combination of iCVD and porous silicon for the development of a controlled drug delivery system. ACS Appl Mater Interfaces 4(7) 3566-3574 Meade SO, Yoon MS, Ahn KH, Sailor MJ (2004) Porous silicon photonic crystals as encoded microcarriers. Adv Mater 16(20) 1811-1814... 

Lauerhaas JM, Credo GM, Heinrich JL, Sailor MJ (1992) Reversible luminescence quenching of porous silicon by solvents. J Am Chem Soc 114(5) 1911-1912 Lazzerini GM, Strambini LM, Barillaro G (2013) Addressing reliability and degradation of chemitransistor sensors by electrical tuning of the sensitivity. Sci Rep 3 1161 Lee EJ, Ha JS, Sailor MJ (1995) Photoderivatization of the surface of luminescent porous silicon with formic acid. J Am Chem Soc 117 8295-8296 Letant SE, Sailor MJ (2000) Detection of HF gas with a porous silicon interferometer. Adv Mater 12(5) 355-359... 

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