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Imperial Journal of Interdisciplinary Research (IJIR)
Vol-2, Issue-11, 2016
ISSN: 2454-1362, http://www.onlinejournal.in
Porous Silicon and its Nanoparticle as
Biomaterial: A Review
1,3
Binoy Bera1, Dipankar Mandal2 & Madhumita Das Sarkar3
Dept. of Computer Science and Engineering, West Bengal University of Technology,
Kolkata – 700064, India
2
Department of Physics, Jadavpur University, Kolkata - 700032, India
Abstract: Porous silicon and its nanoparticles are
receiving growing attention from the scientific
biomedical community due to their favourable
biocompatiability, biodegradability and high surface
area. These nanostructured materials have emerged
as promising multifunctional and versatile platforms
for nanomedicine in drug delivery, diagnostics and
therapy. In this paper a breief review about synthesis
of porous silicon and its nanoparticle and their
application as biomaterial also discussed here.
1. Introduction
Over the past years great advances in
nanotechnology- based platforms have shown
remarkable improvements towards more advanced
delivery systems in order to efficiently direct the
drug molecules to unhealthy tissues or cells. Now a
days, many nanoparticles are used in the clinic and
the development of nanodelivery systems may
accomodate single - or multifunctionalities on the
same entity. However, the biological barriers are
very heterogeneous, which may prevent the
therapeutic and imaging agents from reaching their
intended targets in sufficient amounts. Threfore
shophisticated delivery systems are emerging in
order to develop multimodular nanoassemblies, in
which different components with specific functions
may act in a synergistic manner.
The unique physicochemical properties, as well as
potential biomedical applications of nanomaterials
have long been a subject of intense research
worldwide. Nanomaterials are engineered structures
with at least one dimension below 100 nm,
encompassing highly advantageous features in terms
of mechanical, electrical, chemical and optical
features unlike their bulk materials. According to the
size, morphology/shape, chemical composition and
surface chemistry, nanoparticulate systems may have
diffrent stabilitie and behaviour in the biological
microenvironment and cellular distribution, as well
as elicit undesired biological or toxicological effects.
Therefore, safety and biocompatiability of such
nanomaterials are crucial requirements both in the
Imperial Journal of Interdisciplinary Research (IJIR)
manufacture of the dosage forms and in the
treatement of patients. Consequently, a full
characterization of the in vitro and in vivo behaviour
of such nanomaterials is of utmost importance in the
preclinical assessments.
Porous silicon, a unique “derivative” of silicon,
has a structure of void pores mixed with
microcrystalline and/or nanocrystalline silicon.
Porous silicon is a versatile material for
microelectronics applications. It was first being
turned to practical use for device isolation in 1969 by
the Nipon Telegraph and Telephone Public
Corporation and Sony Corporation. Extremly high
chemical reactivity of porous silicon particularly its
rapid oxidation is very important property. It is
possible to obtain porous silicon through stainetching with hydrofluoric acid, nitric acid and water.
A publication in 1957 revealed that stain films can be
grown in dilute solutions of nitric acid in
concentrated hydrofluoric acid. Porous silicon
formation by stainetching is particularly attractive
because of its simplicity and the presence of readily
available corrosive reagents; namely nitric acid
(HNO3) and hydrogen fluoride (HF). Furthermore,
stain etching is useful if one needs to produce a very
thin porous Si films.
Porous Si has been investigated for applications
in microelectronics, optoelectronics, chemical and
biological sensors, and biomedical devices. The in
vivo use of porous Si was first promoted by Leigh
Canham, who demonstrated its resorbability and
biocompatibility in the mid 1990s. Subsequently,
porous Si or porous SiO2 (prepared from porous Si
by oxidation) host matrices have been employed to
demonstrate in-vitro release of the steroid
dexamethasone, ibuprofen, cisplatin, doxorubicin
,and many other drugs .The first report of drug
delivery from porous Si across a cellular barrier was
performed with insulin, delivered across monolayers
of Caco-2 cells. An excellent review of the potential
for use of porous Si in various drug delivery
applications has recently appeared .An emerging
theme in porous Si as applied to medicine has been
the construction of microparticles (“mother ships”)
with sizes on the order of 1–100 μm that can carry a
molecular or nano sized payload, typically a drug.
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Imperial Journal of Interdisciplinary Research (IJIR)
Vol-2, Issue-11, 2016
ISSN: 2454-1362, http://www.onlinejournal.in
With a free volume that can be in excess of 80%,
porous Si can carry cargo such as proteins, enzymes ,
drugs or genes. It can also carry nanoparticles, which
can be equipped with additional homing devices,
sensors, or cargoes.
In addition, the optical properties of
nanocrystalline silicon can be recruited to perform
various therapeutic or diagnostic tasks—for example,
quantum confined silicon nanostructures can act as
photosensitizers to produce singlet oxygen as a
photodynamic therapy. A long-term goal is to
harness the optical, electronic, and chemical
properties of porous Si that can allow the particles to
home to diseased tissues such as tumors and then
perform various tasks in vivo. These tasks include
detecting, identifying, imaging, and delivering
therapies to the tissue of interest.
.
occurring simultaneously in an electrochemical cell,
the anode (oxidation) reaction and the cathode
(reduction) reaction. Electrochemists refer to these as
“ half - reactions. ” A schematic of a two - electrode
cell for etching silicon, with the relevant half reactions, is shown in Figure 1. The fabrication
process of pSi structures by electrochemical etching
is a cost-competitive and versatile method, by means
of which a variety of porous structures with
outstanding physical and chemical properties can be
produced. This has turned Porous Si into one of the
most widespread materials, which is currently
present in many research fields with multiple
applications. This section deals with the different
aspects involved in the production of macroporous
Si, mesoporous Si and microporous Si structures by
electrochemical etching of silicon wafers.
2. Synthesis of porous silicon and porous
silicon nanoparticle
2.1. Porous Silicon Preparation
For synthesis of porous silicon and its
nanoparticles, first we have to clean silicon wafer. It
is cleaned using the standard RCA cleaning
technique. Silicon (Si) wafers are subjected to
acetone treatment in ultrasonic agitation for 5
minutes followed by rinsing in De-Ionized water.
The substrates are thereafter immersed into strong
piranha etch solution containing Sulphuric Acid (98
% GR) and hrdogen peroxide ( 30% GR) in 3:1
volume ratio for 15 minutes to remove the organic
contaminants present in the wafer. Finally the wafer
is dipped into 10 % HF solution (48 % GR) for 3
minutes for removing native oxide as well as
maintaining surface roughness. De-Ionized water
(Millipore) of resistivity 18.2 M Ω-cm was used to
rinse the sample after each step.
Porous silicon can be prepared by different methods:
1. Electrochemical etching.
2. Strain etch process.
3. Metal assisted strain etch.
4. Vapour phase strain etch.
2.1.1. Electrochemical Etching
In an electrochemical reaction, two electrodes are
needed. One supplies electrons to the solution (the
cathode) and the other removes electrons from the
solution (the anode). It is important to keep in mind
that the two electrodes are required to maintain
charge neutrality and to complete the electrical
circuit. Regardless of the oxidation or reduction
reactions occurring at the electrodes, you cannot
perform electrochemistry if you do not complete the
circuit. This means that at least two reactions are
Imperial Journal of Interdisciplinary Research (IJIR)
Fig.1. Schematic of a two - electrode electrochemical cell
used to make porous silicon. Silicon is the working
electrode. The working electrode is an anode in this case,
because an oxidation reaction occurs at its surface. The
cathode counter - electrode is typically platinum. The main
oxidation and reduction half - reactions occurring during
the formation of porous silicon are given.
Two - Electrode Cell: In the two - electrode cell,
electrochemical reactions must occur at both
electrodes, but generally you are interested in the
reaction at only one of these. In the case of porous
silicon formation, the silicon electrode is the
important one. It is the anode, and the chemical
being oxidized is the silicon itself. The cathode used
in porous silicon etching cells is usually platinum,
and it is separated from the silicon electrode by a few
mm to several cm of electrolyte solution, or in some
cases by a membrane or salt bridge. The
electrochemical reaction occurring at the platinum
electrode is primarily the reduction of protons to
hydrogen gas. Electrochemists refer to the silicon
electrode as the “ working electrode ” , and the
platinum electrode as the “ counter - electrode ” in
this experiment. We are generally not concerned with
the counter – electrode (cathode) reaction, although it
can produce byproducts that interfere with the silicon
electrocorrosion reaction or otherwise limit the
silicon etching process.
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Imperial Journal of Interdisciplinary Research (IJIR)
Vol-2, Issue-11, 2016
ISSN: 2454-1362, http://www.onlinejournal.in
Three - Electrode Cell: A three - electrode
electrochemical cell is used when one wants to
measure both the current and the potential of an
electrochemical reaction simultaneously. This
situation
is
commonly
encountered
in
electrochemistry, because it allows the identification
of the driving force of an electrochemical process. In
a three - electrode configuration, a high - impedance
reference electrode is connected into a specialized
feedback circuit. The reference electrode is designed
to precisely control the electrochemical processes
that can occur at its surface, so that the
electrochemical potential is well defined and does
not drift. A common reference electrode used in
aqueous systems is the saturated calomel electrode. It
is defined by the half - reaction of mercury metal
(Hg) with mercury (I) chloride (Hg2Cl2 ) in a
solution saturated with potassium chloride (KCl).
The mercury, mercury (I) chloride, and potassium
chloride are all contained in a small tube separated
from the electrochemical cell by a porous glass
membrane (typically Vycor glass). The presence of
the membrane and the high impedance (resistance to
current flow) established by the measurement
circuitry ensure that only a small ion current is
allowed to flow between the reference electrode
compartment and the rest of the electrochemical cell,
maintaining a stable reference potential. This sort of
cell is not compatible with solutions containing HF,
and so various other reference electrode
configurations have been used for porous silicon
experiments. A common reference electrode for HF
systems is a bare platinum wire placed in a solution
of HF, separated from the main electrochemical
compartment by a thin plastic capillary tube. This is
not a particularly reproducible reference electrode,
since the electrochemical reaction potentials are
sensitive to surface impurities on the Pt wire.
Nevertheless, the electrode potential is fairly stable
over periods of hours, and it is common practice to
use such electrodes, referred to as “ pseudo reference ” electrodes to indicate their tenuous
relationship to true thermodynamic potentials.
Throughout this book we are less concerned with the
potential at the silicon surface and more concerned
with the total current flowing through it. This is
because most of the key properties of a porous
silicon film: the porosity, pore size, and thickness are
determined by the current. A two – electrode
configuration is sufficient to set this parameter.
2.1.2. Strain Etch Process
It is possible to obtain porous silicon through
stain-etching with hydrofluoric acid, nitric acid and
water. A publication in 1957 revealed that stain films
can be grown in dilute solutions
of nitric acid in concentrated hydrofluoric acid.
Porous silicon formation by stain-etching is
Imperial Journal of Interdisciplinary Research (IJIR)
particularly attractive because of its simplicity and
the presence of readily available corrosive reagents;
namely nitric acid (HNO3) and hydrogen fluoride
(HF). Furthermore, stain-etching is useful if one
needs to produce a very thin porous Si films. A
publication in 1960 by R. J. Archer revealed that it is
possible to create stain films as thin as 25 Å through
stain-etching with HF-HNO3 solution.
Pore Formation Mechanism: The PS formation
mechanism is carried out through etching that can be
considered as localized electrochemical process.
Tiny local anode and cathode sites form on the
etched surface where local cell currents flows
between the sites during etching process. Hence
surface of excess holes and electrons are
incorporated for charge transfer between the tiny
electrodes. Dissolution of silicon take place at the
local anode whereas at local cathode HNO3
reduction is carried out causing holes to be injected
into the silicon. The reaction that takes place at the
local anodic/cathodic sites is:
Cathode: HNO3 + H+ NO + 2H2O + 3h+
Anode: Si + 2H2O + nh+ Sio2 + 4H+ + (4-n)e
Sio2 + 6HF H2SiF6 + 2H20
Where n is the average number of holes required to
dissociate one Si atom. During the etching process
HNO2 acts as the oxidizing agent which is an
intermediate between HNO3 to NO. PS formation
involves the generation of HNO2 from HNO3 as a
rate determining step during the initial stages of the
chemical reaction. HNO2 oxidizes Si to form the
water soluble H2SiF6 and hence Si is etched at the
local anode. As a result PS films are prepared from
Si substrates with different types of doping and
values of resistivity.
2.1.3 Metal Assisted Strain Etch
In a typical metal assisted chemical etching
procedure, a Si substrate partly covered by a noble
metal subjected to an etchant composed of HF and an
oxidative agent. Typically, the Si beneath the noble
metal is etched much faster than the Si without noble
metal coverage. As a result, the noble metal sinks
into the Si substrate, generating pores in the Si
substrate or, additionally, Si wires. The detailed
geometries of the resulting Si structures depend
mostly on the initial morphology of the noble metal
coverage. The first demonstration of metal assisted
chemical etching of Si was reported in 1997. Porous
Si was fabricated by etching an aluminium covered
Si substrate in a solution composed of HF, HNO3,
and H20.
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Imperial Journal of Interdisciplinary Research (IJIR)
Vol-2, Issue-11, 2016
ISSN: 2454-1362, http://www.onlinejournal.in
Pore Formation Mechanism: The following
reactions take place when Al film coated wafer is
subjected to strain etching.
Al + 3HNO3 Al(NO3)3 + 3H+
(1)
Al dissolution takes place on H+ evolution thereby
initiating the oxidation of Si
undergoing the following reaction:
HNO3 + H+ NO + 2H2O +3h+
(2)
Si + 2H2O + nh+ Sio2 + 4H+ + (4-n) e(3)
The Sio2 subsequently reacts with HF to form water
– soluble H2SiF6 by undergoing the
following reaction:
SiO2 + 6HF H2SiF6 + 2H2O
(4)
Hence silicon is etched from the surface by utilising
the Al film deposited on it.
2.1.4 Vapor Phase Strain Etch
Here the PS formation has been done by a
container of 100ml, etching time of 4 min but
varying different oxidation ratio such as- HF:HNO3
(4:1,6:1,8:1). PS fabrication were carried out by the
technique where silicon substrates where exposed to
an etch vapor resulting from the reaction of Zn metal
dust (99.999% pure) continuously added to HF:
HNO3 (oxidation ratio 4:1) acidic solution. An
exposure time of 4 min to these acidic vapors
resulted in the formation of PS structures. Therefore
the wafer kept into the filter paper. These specimens
immediately after etching showed homogeneous PS
films with white crystalline structures observed in
naked eye. But uniform pore is not observed by UVVIS spectrophotometer. Homogeneous PS layers
formed by this technique exhibited not so bright
orange luminescence under UV light.
2.2 Porous Silicon Nanoparticle Preparation
Porous silicon nanoparticle can be prepared by
different methods
2.2.1 Ball Grinding Process
Porous silicon samples have been reduced in
nanometric particles by a well known industrial
mechanical process, the ball grinding in a planetary
mill. The milling experiments were carried out in a
FRITSCH Pulverisette 6 planetary ball mill equipped
with 80 mL agate vial and grinding balls of 10 mm
diameter. The planetary ball mill owes its name to
planet-like movements of the vial: the vessel rotates
around the mill’s central axis and, at the same time,
around its own axis in the opposite direction. The
movement of grinding balls inside the vial ensures
continuous impact with the material. Millimetric
pieces of porous silicon samples and of crystalline
silicon, for
comparison purposes, previously subjected to a
coarse grinding in an agate mortar, were placed in
the vial and then sealed by a clamp. After each
milling run, we collected the powders and
characterized the samples by X-ray powder
diffraction (XRPD), Fourier-transform infrared
spectroscopy (FT-IR), BET surface analysis and
transmission electron microscopy (TEM) in order to
get information on crystallites sizes, and their
physical and chemical properties.
2.2.2 Sacrificial Technique
Once silicon has been made porous, it is removed
in diluted hydroxide solutions (KOH, NaOH,
NH4OH, etc) and, because of its high surface area,
dissolves very quickly even at room temperature .
KOH concentrations as low as 1%, at room
temperature is used to remove porous silicon layers .
Care must be taken to keep the etch rate slow enough
so that the reaction does not become violent, causing
delicate microstructures to be destroyed by bubbles.
2.2.3 Ultrasonification Technique
Fig.2. Vapour phase strain etch
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Porous silicon films are separated from substrate
by applying current density of 50 mA/cm2 for 15
seconds in step two parameters were fixed after
optimization to ensure the separation of monolayer
PS film from Si Substrate this method approximately
7 freestanding obtained from single 250 μm thick
freestanding films so obtained are manually crushed
and then added to DI water to undergo the ultra
process. This solution is then sonicated in a
sonication bath at 42 kHz frequency for 4,6,10 and
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Imperial Journal of Interdisciplinary Research (IJIR)
Vol-2, Issue-11, 2016
ISSN: 2454-1362, http://www.onlinejournal.in
12 hours and the samples are labeled as 4Hr, 6Hr,
10Hr and 12Hr.
3. Porosity of porous silicon and
nanoparticle size measurement
the application of porous silicon and porous
silicon nanoparticle as biomaterial it is important to
know the porosity of porous silicon sample and
nanoparticle size of porous silicon nanoparticles.
3.1. Porosity Measurement
Porosity defined as ratio of the total pore volume
Vp to the apparent volume V.
Porosity(%) =
Where
m1 = weight of silicon before the etching.
m2 = weight of the silicon after the etching.
m3 = weight of the wafer silicon after NPS removed.
3.2. Nanoparticle Size Measurement
Nanoparticle size can be measured by Dynamic
light scattering. Dynamic Light Scattering is also
known as Photon Correlation Spectroscopy. This
technique is one of the most popular methods used to
determine the size of particles. Shining a
monochromatic light beam, such as a laser, onto a
solution with spherical particles in Brownian motion
causes a Doppler Shift when the light hits the
moving particle, changing the wavelength of the
incoming light. This change is related to the size of
the particle. It is possible to compute the sphere size
distribution and give a description of the particle’s
motion in the medium, measuring the diffusion
coefficient of the particle and using the
autocorrelation function. This method has several
advantages: first of all the experiment duration is
short and it is almost all automatized so that for
routine measurements an extensive experience is not
required. Moreover, this method has modest
development costs. Commercial "particle sizing"
systems mostly operate at only one angle (90°) and
use red light (675 nm). Usually in these systems the
dependence on concentration is neglected.
Imperial Journal of Interdisciplinary Research (IJIR)
Fig.3. Dynamic Light Scattering Method
4. Application as biomaterial
Due to its biodegradability, low toxicity, large pore
volume, tunable pore size, and intrinsic
photoluminescence, porous silicon has shown
considerable potential for biomedical applications. In
1995, Canham first demonstrated that porous silicon
film can induce hydroxyapatite growth on its surface
in simulated body fluid, suggesting the potential use
of porous silicon as bioactive material for tissue
engineering. Lin et al showed that porous silicon
chips could be used as low power biosensors to
detect picomolar DNA oligomers or proteins. Cheng
et al studied the longterm toxicity and degradation of
porous silicon microparticles in vitreous humor . The
results show the microparticles have good
biocompatibility and stability, which suggests the
material can be used as intraocular drug delivery
system. A first human study using radioactive
Phosphorus-32 (32P) doped porous silicon
microparticles as intratumoral implants to treat liver
cancer showed that the treatment was safe and well
tolerated. An antitumor response was also observed
in the early stage of this clinical study. For oral drug
delivery, Salonen and Foraker et al demonstrated that
porous silicon microparticles could be used to carry
poorly water- drugs, protect them from biological
environments, and better delivery the drug molecules
across intestinal cell monolayers. Recently, Low et al
studied the biocompatibility of various types of
porous silicon membranes in tissues of the eye. They
found thermally-oxidised, aminosilanised porous
silicon membrane elicited very little host reaction
following implantation into the rat eye and the
material supported the attachment and growth of
human ocular cells. Their finding suggests porous
silicon can be potentially used to deliver cells to the
ocular surface as a treatment for corneal epithelial
stem cell dysfunction. RNA interference (RNAi) is a
powerful approach for silencing genes associated
with a variety of pathologic conditions. However, in
vivo delivery of interfering RNA has been a
challenge due to lack of safe, efficient, and sustained
system. The delivery of small interfering RNA
(siRNA) using porous silicon has been demonstrated
recently. Porous silicon macroparticles loaded with
liposomes containing siRNA against an oncogene
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Imperial Journal of Interdisciplinary Research (IJIR)
Vol-2, Issue-11, 2016
ISSN: 2454-1362, http://www.onlinejournal.in
resulted in sustained gene silencing for 3 weeks in
ovarian cancer model.
[6] Lin, V. S.-Y.; Motesharei, K.; Dancil, K. P. S.; Sailor,
M. J.; Ghadiri, M. R. A porous silicon-based optical
interferometric biosensor. Science 1997, 278, 840–843.
5. Conclusion
[7] Conibeer G, Green M, Corkish R, Cho Y, Cho E C,
Jiang C W, Fang-suwannarak T, Pink E, Huang Y D,
Puzzer T, Trupke T, Richards B,Shalav A and Lin K L 2006
Thin Solid Films 511 65.
Particle sizing is a very important factor in the field
of biomedical application (such as drug delivery). By
tuning the pore sizes of porous silicon, different size
of porous particles can be gained. By dynamic light
scattering the quality of the sample can be observe
(i.e. whether the sample is poly disperse or mono
disperse), from there the idea about the shape, size
and mass of the particle can be obtained. Cancer is
the second leading cause of death, claiming ~ 0.56
million lives in the U.S. every year following heart
diseases ( ~ 0.62 million). From 1991 to 2007,
mortality associated with heart diseases decreased
39%; by contrast, the death rate of cancer only
decreased by 17% in spite of intensive research and
improved therapeutics. Porous silicon nanoparticle is
used as cancer diagnostic and therapeutic agents.
Folic acid insufficiency has long been related to the
occurrence of various diseases. However, the loss of
integrity of folic acid has led to the investigation of
strategies to improve the vitamin stability and
controlled release. Porous silicon nanoparticle is an
attractive inorganic material for drug delivery
applications due to its biocompatibility and tunable
degradation behavior.
6. Acknowledgements
Binoy Bera would like to thank Dr. Dipankar Mandal
and Dr. Madhumita Das Sarkar for their constant
support, inspiration and guidance.
[8] E.J. Anglin, L. Cheng, W.R. Freeman, and M.J. Sailor,
Adv. Drug Delivery Rev., 60 (2008) 1266- 1277.
[9] Yuriy Vashpanov 1, Jung Young Son 2,* and Kae Dal
Kwack Mesoporous Silicon with Modified Surface for
Plant Viruses and Their Protein Particle Sensing Sensors
2008, 8, 6225- 6234; DOI: 10.3390/s8106225.
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kristallisationsgeschwindigheit der metalle. Z. Phys. Chem.
92, 219–221 (1918).
[11] J. Kilby, Invention of the integrated circuit. IEEE
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[12] A. Jr, Uhlir, Electrolytic shaping of germanium and
silicon. The Bell Syst. Tech. J. 35, 333– 347 (1956).
[13] C.S. Fuller, J.A. Ditzenberger, Diffusion of donor
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[14] D.R. Turner, Electropolishing silicon in hydrofluoric
acid solutions. J. Electrochem. Soc. 105, 402–408 (1958).
[15] P.F. Schmidt, D.A. Keiper, On the jet etching of ntype Si. J. Electrochem. Soc. 106, 592– 596 (1959).
[16] R.J. Archer, Stain films on silicon. J. Phys. Chem.
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D.R. Turner, in The electrochemistry of
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7. References
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