Porous Silicon in Practice: Preparation, Characterization and Applications

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This is a dummy description. By means of electrochemical treatment, crystalline silicon can be permeated with tiny, nanostructured pores that entirely change the characteristics and properties of the material. One prominent example of this can be seen in the interaction of porous silicon with living cells, which can be totally unwilling to settle on smooth silicon surfaces but readily adhere to porous silicon, giving rise to great hopes for such future applications as programmable drug delivery or advanced, braincontrolled prosthetics.

Porous silicon research is active in the fields of sensors, tissue engineering, medical therapeutics and diagnostics, photovoltaics, rechargeable batteries, energetic materials, photonics, and MEMS Micro Electro Mechanical Systems. Written by an outstanding, well-recognized expert in the field, this book provides detailed, step-by-step instructions to prepare and characterize the major types of porous silicon. It is intended for those new to the fi eld.

Porous Silicon in Practice: Preparation, Characterization and Applications by Michael J. Sailor

For materials scientists, chemists, physicists, optical physicists, biomaterials scientists, neurobiologists, bioengineers, and graduate students in those fields, as well as those working in the semiconductor industry. About the Author Michael J.

Sailor has been working in the porous silicon field for 20 years. He received a B. In this experimental study, PSi pore sizes on the PSi surface were controlled for optimum filling with DNA oligonucleotides and surface roughness was optimized for obtaining higher resolution RIS patterns. In addition, the influence of two different organic electrolyte mixtures on the formation and morphology of the pores, based on various current densities and etching times on doped p-type silicon, were examined.

Moreover, we introduce two cleaning processes which can efficiently remove the undesirable outer parasitic layer created during PSi formation. Results of all the optimization steps were observed by field emission scanning electron microscopy FE-SEM. DNA sensing reached its optimum when PSi was formed in a two-step process in the ethanol electrolyte accompanied by removal of the parasitic layer in NaOH solution.

These optimal conditions, which result in pore sizes of approximately 20 nm as well as a low surface roughness, provide a considerable RIS shift upon complementary sequence hybridization, suggesting efficient detectability. We demonstrate that the optimal conditions identified here makes PSi an attractive solid-phase DNA-based biosensing method and may be used to not only detect full complementary DNA sequences, but it may also be used for detecting point mutations such as single nucleotide substitutions and indels. Porous silicon PSi , a nanostructure of silicon material with a high surface-to-volume ratio, versatile surface chemistry, and adjustable morphology and pore diameters, has been commonly used in the medical and therapeutic fields especially in fabricating sensors and biosensors 1 , 2.

In biosensing applications, PSi can be used as a suitable transducer in combination with a variety of detection methods including those based on electrical, electrochemical, optical and thermal methods 3 - 5. Since then, due to its applicability as a label- free biosensor, this method has received considerable attention 5 , 7 - Where m is the spectral order of the fringe and 2nd is the effective optical thickness EOT, twice of the product of the refractive index n and the thickness d of the layer 18 , Upon applying a solution containing biomolecules to the PSi layers, the pores of PSi are filled with such biomolecules, which in turn cause variation in the refractive index.

Consequently, the observed shift in the interference pattern may be used as a robust signal for biosensing applications. First, the pore sizes of PSi must be sufficiently large to allow biomolecules to penetrate the pores freely.

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However, pores larger than a certain limit decrease sensitivity due to the reduction in surface area. Therefore, PSi pore size optimization is essential in accurately sensing biomolecules. This increases the mirror reflection and interference pattern becomes significantly visible 20 , BRCA1 is the most highly-penetrant breast cancer susceptibility gene in breast and ovarian cancer Detection of BRCA1 at both genomic and transcriptomic levels is useful in breast and ovarian cancer diagnosis and its screening may identify individuals with a high risk of cancer development Given that each nucleotide is approximately 0.

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In this experimental study, p-type silicon wafers with resistivity of 0. Prior to the electrochemical process, a simple cleaning procedure was undertaken [by soap, deionized DI water and ethanol].

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However, in some cases, after this process, samples were placed inside the electrolyte [consisting of a mixture of wt. Afterwards, they were sonicated successively in chloroform, acetone and ethanol respectively for 10 minutes. After the cleaning process, silicon substrates were placed inside an electrochemical cell and the electrochemical process was undertaken by two types of novel electrolyte mixtures.

The first electrolyte was a wt. The current density and etching time in preparing samples based on this electrolyte are given in Table 1. The next electrolyte consisted of a mixture of wt. The samples were treated under different electrochemical etching conditions with the electrochemical process being undertaken in two steps on two E-samples Table 2 ,. The HF reaction in the electrochemical process causes the prepared PSi surface to become extremely active with H-bond formations e.

These H-bonds are not stable in environmental conditions due to the exchange with oxygen groups, resulting in surface oxidation The wafers were then rinsed with deionized water and subsequently dried. Finally, the prepared samples were rinsed with deionized water three times to remove excess and mobile DNA 27 , After the incubation period, samples were rinsed with deionized water to remove unhybridized DNA. RIS was implemented by using a tungsten lamp illuminating the surface through an optical fibre. A collimator was then used to collect the reflected beam using an objective lens coupled with a multimode fibre which was directed into a spectrophotometer EPPHR with a spectral resolution of 0.

Briefly, the reflected spectrum was an interference pattern containing successive maxima and minima. PSi surface analysis and spectroscopic characterization of each PSi sample is presented separately below. Since surface roughness and distribution of pore diameters are important for biosensing detection, FESEM images of the samples were taken to assess the effect of electrolyte media and applied currents on these two factors.

In sample 1D, based on a current density of 8. Also, bigger cavities were observed of which some were distributed non-uniformly. It is possible that the walls between some cavities have been destroyed and thus resulting in larger merge cavities. The current density was then reduced to 3. This reduced pore sizes to around nm along with a uniform distribution. In sample 3D, we continued the optimization process by applying a lower current density at 1.

Smaller pores in the range of nm were formed Fig.

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By halving the current density to 0. Images were then taken from the cross-section to analyse roughness of PSi samples Fig. This level of roughness in PSi samples which are anodized by the DMF electrolyte is in agreement with our previous study ImageJ was used to quantify the size distribution of the pores according to the applied current densities Fig. The decrease in current density from 8.

Porous Silicon humidity effect

In addition, at a lower current density, a more uniform size distribution was observed. The obtained spectra were measured at wavelengths between and nm. At first, we measured the reflectance spectrum of the silicon substrate Fig. S1 , See Supplementary Online Information at www. The same measurement was undertaken for sample 2D and the same peak at the wavelength of nm was observed Fig. The reflectance spectra of the rest of the D-samples did not show any meaningful interference pattern. In this section, we present the characterizations of E-samples prepared in the ethanol electrolyte.

Uniform surface with very small pores of about 5 to 30 nm was formed on the surface. Next, by increasing the current density in sample 3E, the outermost surface showed no change in porosity compare the main plot of Fig. In addition, D increasing current density can cause the appearance of scab on the surface. By going inside the scab, the SEM images showed that the underneath layer has a different porosity Fig.

Also, images taken from the cross-section of samples 2E Fig. Image analysis showed that the thickness of both parasitic and main layers were grown by increasing the current density Fig. S2 , See Supplementary Online Information at www. However, in these samples, formation of cracks is more dominant compare insets of Fig. Thus, afterNaOH treatment, which removes PSi layers both of parasitic and main layers , the silicon surface would remain rough.

Therefore, in the second step electrochemical etching, pore formation was initiated based on the rough background. However, by decreasing the current density and the time of etching in the first step, suitable pore diameters were grown on the surface with no parasitic layer Fig. RIS was undertaken for wavelengths between nm. The 4E sample did not show any interference pattern.

RIS was then implemented for 3E and 5E samples. The reflectometry measurement based on 3E and 5E samples showed both interference pattern Fig. Since EOT is equal to 2nl [twice of the product of the refractive index n and the thickness d ], any change in porosity and consequently refractive index or thickness alter the EOT value. Thus, the reproducibility of extracted EOT was used as representative of the reproducibility of these samples. Although FE-SEM images of replicate samples could have been used to check reproducibility, the EOT method was not only more cost-effective and convenient, but it was also readily quantifiable.

After each functionalization step, these characteristics interference pattern remained which is illustrated between nm for the 3E and 5E samples after immobilization of the DNA probe in Figure 4B. After hybridization of the target DNA, the interference patterns of both 3E and 5E samples were shifted to the larger wavelengths i. However, the red shift for the 5E sample was larger than the 3E sample. We used non-complementary target DNAs for both samples 3E and 5E after probe immobilisation but did not observe any variation in the extracted EOT data not shown , suggesting that the extracted EOT are true representatives of the interference signal from the hybridization of complementary target probes.

For the RIS method to work efficiently as a new transducer in biosensing on PSi substrates, two conditions should be met. In general, the specular reflectance at normal incidence is given by equation 2 21 , 33 , 34 :. Based on equation 2, the specular reflectance is inversely proportional to the roughness of the surface.

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S3 , See Supplementary Online Information at www. Secondly, the pore sizes of PSi must be optimum. Because of the reduction in surface area and consequently reduction in sensitivity, large pores are not desirable. Small pores which do not permit biomolecules to penetrate the pores freely are also not useful. The pore sizes could be easily controlled by adjusting the current density in the electrochemical process in D samples. However, the high surface roughness of these samples is an obstacle. On the other hand, the formation of the crust layer in E samples which are reported previously 19 , 31 , 32 , 36 - 40 is also a serious problem.

This crust layer plays as a parasitic layer and can prohibit the diffusion of biomolecules into the main layer.

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Therefore, applying some removal techniques were one of the challenges in our work which is also previously studied and reported for different techniques by others 19 , 32 , 39 , Finally, by using a controlled two-step process in the ethanol electrolyte accompanied by removal of the parasitic layer in NaOH solution, we were able to satisfy the two essential conditions for PSi substrates.

As a consequence, DNA sensing reached its optimum with these samples, where pore sizes were approximately 20 nm wide and surface roughness was as low as possible. Upon complementary sequence hybridization, we observed a considerable RIS shift, suggesting efficient detectability of DNA molecules.


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