Protein Adsorption in Thin Porous Silicon Layers

Mesoporous Silicon Particles Favor the Induction of Long-Lived Humoral Responses in Mice to a Peptide-Based Vaccine

Vaccinology faces the challenge of developing improved immunization approaches that are able to induce long-term immunity with the desired Th profile according to the pathology. In this context, new vehicles for efficient antigen delivery that exert adjuvant effects play a critical role in addressing this goal. Herein, mesoporous silicon particles (PSiP) were assessed as carriers for a peptide-based vaccine targeting the receptor for advanced glycation end products (RAGE), which is a relevant receptor in Alzheimer´s disease and other diseases. A RAGE peptide was adsorbed onto PSiP (PSiP vaccine) and administered to BALB/c mice, leading to immune responses that were similar in magnitude to those induced by the soluble peptide. However, the response induced by PSiP lasted for a significantly longer period when compared with the behavior of the group immunized with the peptide alone. Therefore, PSiP are proposed as carriers to enhance immune memory, which is critical in vaccination. This study opens interesting perspectives related to the application of PSiP in vaccinology.

Protection and Delivery of Anthelmintic Protein Cry5B to Nematodes Using Mesoporous Silicon Particles

The ability of nano- and microparticles of partially oxidized mesoporous silicon (pSi) to sequester, protect, and deliver the anthelmintic pore-forming protein Cry5B to nematodes is assessed in vitro and in vivo. Thermally oxidized pSi particles are stable under gastric conditions and show relatively low toxicity to nematodes. Fluorescence images of rhodamine-labeled pSi particles within the nematodes Caenorhabditis elegans and Ancylostoma ceylanicum show that ingestion is dependent on particle size: particles of a 0.4 ± 0.2 μm size are noticeably ingested by both species within 2 h of introduction in vitro, whereas 5 ± 2 μm particles are excluded from C. elegans but enter the pharynx region of A. ceylanicum after 24 h. The anthelmintic protein Cry5B, a pore-forming crystal (Cry) protein derived from Bacillus thuringiensis, is incorporated into the pSi particles by aqueous infiltration. Feeding of Cry5B-loaded pSi particles to C. elegans leads to significant intoxication of the nematode. Protein-loaded particles of size 0.4 μm display the highest level of in vitro toxicity toward C. elegans on a drug-mass basis. The porous nanostructure protects Cry5B from hydrolytic and enzymatic (pepsin) degradation in simulated gastric fluid (pH 1.2) for time periods up to 2 h. In vivo experiments with hookworm-infected hamsters show no significant reduction in worm burden with the Cry5B-loaded particles, which is attributed to slow release of the protein from the particles and/or short residence time of the particles in the duodenum of the animal.

A Highly Sensitive Porous Silicon (P-Si)-Based Human Kallikrein 2 (hK2) Immunoassay Platform toward Accurate Diagnosis of Prostate Cancer

Levels of total human kallikrein 2 (hK2), a protein involved the pathology of prostate cancer (PCa), could be used as a biomarker to aid in the diagnosis of this disease. In this study, we report on a porous silicon antibody immunoassay platform for the detection of serum levels of total hK2. The surface of porous silicon has a 3-dimensional macro- and nanoporous structure, which offers a large binding capacity for capturing probe molecules. The tailored pore size of the porous silicon also allows efficient immobilization of antibodies by surface adsorption, and does not require chemical immobilization. Monoclonal hK2 capture antibody (6B7) was dispensed onto P-Si chip using a piezoelectric dispenser. In total 13 × 13 arrays (169 spots) were spotted on the chip with its single spot volume of 300 pL. For an optimization of capture antibody condition, we firstly performed an immunoassay of the P-Si microarray under a titration series of hK2 in pure buffer (PBS) at three different antibody densities (75, 100 and 145 µg/mL). The best performance of the microarray platform was seen at 100 µg/mL of the capture antibody concentration (LOD was 100 fg/mL). The platform then was subsequently evaluated for a titration series of serum-spiked hK2 samples. The developed platform utilizes only 15 µL of serum per test and the total assay time is about 3 h, including immobilization of the capture antibody. The detection limit of the hK2 assay was 100 fg/mL in PBS buffer and 1 pg/mL in serum with a dynamic range of 10⁶ (10⁻⁴ to 10² ng/mL).

Real-time study of protein adsorption kinetics in porous silicon

This paper presents an optical method for real-time monitoring of protein adsorption using porous silicon self-supported microcavities as a label-free detection platform. The study combines an experimental approach with a physical model for the adsorption process. The proposed model agrees well with experimental observations, and provides information about the kinetics of diffusion and adsorption of proteins within the pores, which will be useful for future experimental designs.

Spectrally resolving single-shot polarimeter

We demonstrate a spectrally resolving single-shot polarimeter. The system consists of a commercial imaging spectrograph, modified by a birefringent wedge and a segmented polarizer. The physical operating principle and limitations of the apparatus as well as preliminary polarimetric measurements on the emission of random lasers are reported.

Thermal oxidation for controlling protein interactions with porous silicon

Thermal oxidation of porous silicon (pSi) has been used to control interactions with three proteins; lysozyme, papain, and human serum albumin (HSA) enabling the influences of protein structure, molecular weight, and charge to be elucidated. Adsorption behavior was assessed via adsorption isotherms while the structures of adsorbed proteins were investigated using a bioactivity assay, FTIR, and zeta potential. Time-of-flight secondary ion mass spectrometry was used to examine protein pore penetration. High protein adsorption onto unoxidized pSi (240-610 microg/m(2)) was attributed to predominately hydrophobic interactions which resulted in structural changes of the adsorbed proteins and significant loss of bioactivity. Thermal oxidation at 400 and 800 degrees C significantly reduced protein adsorption (80-485 microg/m(2)) by reducing hydrophobicity. Oxidation of pSi modified the protein adsorption mechanisms to solely electrostatic attraction for positively charged proteins and structural rearrangement for negatively charged proteins. Adsorption via electrostatic attraction preserved protein bioactivity and zeta potential, thus inferring a retention of their native structure. In contrast, the negative charge and globular structure of HSA resulted in a loss of structure. We have demonstrated that thermal oxidation of pSi can be used to control protein interactions, adsorbed structure, and bioactivity.

Steric crowding effects on target detection in an affinity biosensor

This work quantifies the impact of steric crowding on whole antibody (Ab) receptor immobilization and target Ab detection and also demonstrates how the versatile biotin/streptavidin receptor immobilization system must be tuned to optimize target detection in designing biosensors. Results are demonstrated on a label-free optical biosensor fabricated from n-type macroporous porous silicon (PSi) with approximately 88-107 nm diameter pores. We employ a sandwich assay scheme comprising a linking chemistry (biotin/streptavidin) to attach biotinylated anti-rabbit IgG (receptor) to detect rabbit IgG (target). A "bottom-up" approach was taken to investigate each layer of the sandwich assay to optimize target binding. Steric crowding was observed to hinder subsequent layer binding for each layer in the sandwich (biotin, streptavidin, and receptor). Our results give definitive evidence that the onset of steric crowding within the biotin layer occurs at a surface coverage of 57%, which is much higher compared to that from published work on well-ordered self-assembled biotin monolayers on planar gold surfaces. This difference is attributed to the topographical heterogeneity of the PSi substrate. Streptavidin (SA) binding to surface-linked biotin was altered by preblocking the streptavidin binding sites with biotin. Through consistent trends in data, preblocking SA was shown to reduce steric crowding within the SA layer, which translated into increased receptor immobilization. The final detection range of rabbit IgG was 0.07-3 mg mL(-1) (0.4-17 ng mm(-2)), and binding specificity was demonstrated by employing an anti-chicken IgG control receptor. This study underlines the importance of considering binding avidity and surface topography in optimizing chip-based biosensors.

Delivery of nanogram payloads using magnetic porous silicon microcarriers

A transport and delivery system for nanogram quantities of molecular species that does not use microfluidic channels, pumps, or valves is described. Microparticles consisting of magnetic porous silicon are prepared, and loading and delivery of an enzymatic payload are demonstrated. The high porosity (60%) porous Si host particles are made magnetic by infusion of superparamagnetic iron oxide nanoparticles (30 nm-diameter magnetite, Fe(3)O(4)) under oxidative conditions. After magnetite incorporation, the porous microparticle is still empty enough to accommodate nanogram quantities of a molecular payload; the enzymes horseradish peroxidase or pronase E are used in the present work. The assembly can be transported to a microliter water droplet containing the enzyme substrate with the aid of an external magnetic field. The enzyme is released into the droplet upon contact. The particles can be transported through air or a hydrocarbon liquid without loss in enzymatic activity of the payload.

Integrated protein microchip assay with dual fluorescent- and MALDI read-out

A pore chip protein array (PCPA) concept based on a dual readout configuration, fluorescence imaging, and MALDI-TOF MS has been developed. Highly packed, (>4000 spots/cm2), antibody arrays were dispensed on the porous chip by using a piezo-electric microdispenser. Sandwich assay was made after blocking by addition of a secondary antibody either IgG-FITC-labeled or anti-Ang II. The antigen in the first system was a large protein (IgG), and in the other system, a FITC marked peptide Angiotensin II (Ang II) was used. Ang II antibodies showed specificity for Ang II, while the Ang I antibodies showed binding properties for Ang I, II, and Renin. Fluorescence and MALDI TOF MS read-out was made for IgG and Ang II. A major advantage of the dual read-out PCPA approach is that both affinity binding and mass identity are derived. Detection limits for Ang II on the chip is as low as 500 zmol (Ang II).

Engineering the chemistry and nanostructure of porous silicon Fabry-Pérot films for loading and release of a steroid

A method for engineering the surface chemistry and pore dimensions in porous Si films for the purpose of controlling the loading and release of a hydrophobic drug is described. Loading of the steroid dexamethasone is confirmed by Fourier transform infrared spectroscopy, and the release rates are characterized by observation of the appearance of the drug in solution (UV-vis absorption spectroscopy) and by measurement of the Fabry-Perot fringes in the optical reflectivity spectrum of the porous Si film. Optical reflectivity changes provide a measure of the release rate of the drug that is amenable to in-vivo diagnostic applications. Fresh porous Si films are prepared by electrochemical etch and subsequently modified by hydrosilylation with 1-dodecene. The dodecene-modified samples are more robust in aqueous environments and exhibit slower release rates of the drug relative to freshly etched porous Si. Whereas the relatively large dexamethasone molecule is found to infiltrate the freshly etched samples, it does not enter the chemically modified films, because of steric crowding from the dodecyl species. To achieve a high degree of loading into these modified films, the pores are enlarged before hydrosilylation by treatment with an aqueous solution containing HF and dimethyl sulfoxide. The pore expanded, chemically modified samples admit approximately 70% of the dexamethasone that can be admitted into an unmodified (freshly etched) sample. Diffusion of the steroid from the modified, pore expanded films into phosphate-buffered saline solution is slower than from the unmodified sample by a factor of approximately 20, with 90% of the drug delivered in 3 days for the chemically modified films compared to 3 h for the unmodified films.

Manipulation of liquid droplets using amphiphilic, magnetic one-dimensional photonic crystal chaperones

The controlled manipulation of small volumes of liquids is a challenging problem in microfluidics, and it is a key requirement for many high-throughput analyses and microassays. One-dimensional photonic crystals made from porous silicon have been constructed with amphiphilic properties. When prepared in the form of micrometre-sized particles and placed in a two-phase liquid such as dichloromethane/water, these materials will accumulate and spontaneously align at the interface. Here we show that superparamagnetic nanoparticles of Fe(3)O(4) can be incorporated into the porous nanostructure, allowing the materials to chaperone microlitre-scale liquid droplets when an external magnetic field is applied. The optical reflectivity spectrum of the photonic crystal displays a peak that serves to identify the droplet. Two simple microfluidics applications are demonstrated: filling and draining a chaperoned droplet, and combining two different droplets to perform a chemical reaction. The method provides a general means for manipulating and monitoring small volumes of liquids without the use of pumps, valves or a microfluidic container.

Quantatitive Assessment of Enzyme Immobilization Capacity in Porous Silicon

Immobilized enzyme systems are important in a broad range of applications, from biological sensing to the industrial-scale biocatalytic synthesis of chiral products. We demonstrate the ability to systematically vary and quantitatively assess the immobilization capacity of porous silicon thin films for the enzyme glutathione-S-transferase in a manner predicted by a simple geometric model of the porous silicon matrix. We find that the immobilization capacity quantatitively correlates with systematic changes in the device thickness. These results are significant since, despite the wide range over which porous silicon morphology and surface area can be varied, few attempts have been made to systematically characterize surface binding capacity. Our findings suggest that porous silicon can be an ideal matrix, where immobilization of a predictable quantity of biological material is desired.

Bovine serum albumin adsorption on passivated porous silicon layers

Hydrogen-terminated porous silicon (pSi-H) films were fabricated through electrochemical anodization of crystalline silicon in hydrofluoric-acid-based solutions. The pSi-H surface was chemically functionalized by thermal reaction with undecylenic acid to produce an organic monolayer covalently attached to the silicon surface through Si—C bonds and bearing an acid terminal group. Bovine serum albumin (BSA) was adsorbed onto such surface-modified pSi structures. The resulting surfaces were characterized using scanning electron microscopy (SEM), reflection FT-IR spectroscopy, and ellipsometry. SEM showed that the porous films were damaged and partially lifted off the silicon substrate after a prolonged BSA adsorption. Ellipsometry analysis revealed that the BSA penetrated ∼1.3 µm into the porous structure. The film damage is likely a result of BSA anchoring itself tightly through strong electrostatic interaction with the acid-covered Si sidewalls. A change in surface tension during BSA film formation then causes the pSi layer to buckle and lift off the underlying Si substrate. FT-IR results from the undecylenic-acid-modified pSi surfaces before and after BSA adsorption showed the presence of strong characteristic amide I, II, and III vibrational bands after BSA adsorption. The surface properties of the pSi matrix and its interactions with BSA are examined in this study.Key words: ellipsometry, porous silicon, protein adsorption, surface passivation.

Penetration and loading of human serum albumin in porous silicon layers with different pore sizes and thicknesses

Human serum albumin was adsorbed into porous silicon layers with thickness up to 3 microm and with different mean pore radius in the range 4.5-10 nm. The adsorbed amount of protein was quantified by I(125) radioactive labeling techniques and ellipsometry. The results show that albumin penetrated into the pores when the mean pore radius was larger than 5.5 nm, but could not totally occupy the available surface area when the layer thickness was larger than 1 microm. Loading of albumin both into porous layers and onto plane silicon as a function of albumin concentration was also investigated. These measurements show that loading of protein increased with protein concentration at least up to 10 mg/ml for porous silicon and up to 1 mg/ml for plane silicon. The maximum deposition into the type of porous layers used here was 28 microg/cm(2), compared to 0.36 microg/cm(2) for plane silicon.

Smart dust: self-assembling, self-orienting photonic crystals of porous Si

Micrometer-sized one-dimensional photonic crystals of porous Si that spontaneously assemble, orient, and sense their local environment are prepared. The photonic crystals are generated by electrochemically etching two discrete porous multilayered dielectric mirrors into Si, one on top of the other. The first mirror is chemically modified by hydrosilylation with dodecene before the etching of the second mirror, which is prepared with an optical reflectivity spectrum that is distinct from the first. The entire film is removed from the substrate, and the second mirror is then selectively modified by mild thermal oxidation. The films are subsequently fractured into small particles by sonication. The chemically asymmetric particles spontaneously align at an organic liquid-water interface, with the hydrophobic side oriented toward the organic phase and the hydrophilic side toward the water. Sensing is accomplished when liquid at the interface infuses into the porous mirrors, inducing predictable shifts in the optical spectra of both mirrors.