Today, with Q-dots as new nanoball bearings, aluminosilicates as nanowire gauges, cochle-ates as nanocrystalline delivery trucks and iron nanoparticles as new biomagnets, it is no wonder
Trang 1Nanostructured metallic films approximately
200–500 nm thick have been a part of ceramic
decorations ‘luster’ since the medieval period
The homogeneous dispersion of silver and
cop-per nanoparticles over glazed pottery results in
a colored iridescence called luster, a technique
very popular in the Middle East, Egypt, Persia
and Spain In 1685, Andreas Cassius invented
a recipe of glass coloring pigment called Purple
of Cassius He made the purple precipitates by
dissolving gold particles in aqua regia and then
added a piece of tin to it The pigment is famous
for its use in high-quality porcelain ware A
Vien-nese chemist, Richard Zsigmondy was awarded
the Nobel Prize in Chemistry in 1925 for
discov-ering that colloidal gold adsorbed on stannous
hydroxide base makes Purple of Cassius Michael
Faraday (1857) prepared gold sols, whose size was
found out by JM Thomas in 1988 to be 3–30 nm
in diameter, when he reproduced those gold
nanoparticles Industrial nanotechnology was
initiated in 1930 with the manufacturing of
sil-ver coatings for photographic films At around
this time imaging techniques such as ultrasound,
MRI, computed tomography, positron emission
tomography and surface enhanced raman spec-troscopy were becoming popular for imaging various disease states and magnetic nanopar-ticles came to the rescue as contrast agent with unique physicochemical properties The 1980s took nanomaterials to higher strata by introduc-ing fullerenes and scannintroduc-ing tunnel microscopes
In the mid-1980s, when growth technologies such as molecular beam epitaxy tied nuptials with electron-beam lithography to confine electron motion in all three (x-y-z) directions, quantum dots (Q-dots) were produced These metallic nanoparticles have been embraced by nanotechnology for more than four decades now
Today, with Q-dots as new nanoball bearings, aluminosilicates as nanowire gauges, cochle-ates as nanocrystalline delivery trucks and iron nanoparticles as new biomagnets, it is no wonder that nanoscience thinks about metals in terms
of size control, spatial resolution, chemical reac-tivity and engineering their relationship at the cellular level in real time
Metal nanoparticles can be easily prepared to the nanometer scale, they possess fundamentals
of light matter interaction and are highly suitable
Metallic nanoparticles and their medicinal
potential Part II: aluminosilicates,
nanobiomagnets, quantum dots and cochleates
Metallic miniaturization techniques have taken metals to nanoscale size where they can display fascinating properties and their potential applications in medicine In recent years, metal nanoparticles such as aluminium, silicon, iron, cadmium, selenium, indium and calcium, which find their presence in aluminosilicates, nanobiomagnets, quantum dots (Q-dots) and cochleates, have caught attention of medical industries The increasing impact of metallic nanoparticles in life sciences has significantly advanced the production techniques for these nanoparticles In this Review, the various methods for the synthesis of nanoparticles are outlined, followed by their physicochemical properties, some recent applications in wound healing, diagnostic imaging, biosensing, assay labeling, antimicrobial activity, cancer therapy and drug delivery are listed, and finally their toxicological impacts are revised The first half
of this article describes the medicinal uses of two noble nanoparticles – gold and silver This Review provides further information on the ability of aluminum, silicon, iron, selenium, indium, calcium and zinc to be used as nanoparticles
in biomedical sciences Aluminosilicates find their utility in wound healing and antibacterial growth Iron-oxide nanoparticles enhance the properties of MRI contrast agents and are also used as biomagnets Cadmium, selenium, tellurium and indium form the core nanostructures of tiny Q-dots used in cellular assay labeling, high-resolution cell imaging and biosensing Cochleates have the bivalent nano ions calcium, magnesium or zinc imbedded in their structures and are considered to be highly effective agents for drug and gene delivery The aluminosilicates, nanobiomagnets, Q-dots and cochleates are discussed in the light of their properties, synthesis and utility
Leena Loomba 1 & Tiziano Scarabelli* 2,3
1 Punjab Agricultural University, Ludhiana, Punjab, India
2 Center for Heart & Vessel Preclinical Studies, St John Hospital & Medical Center, Wayne State University School of Medicine, Detroit, MI, USA
3 Medical Center, Wayne State University School of Medicine,
MI, USA
*Author for correspondence: E-mail: tiziano.scarabelli@wayne.edu
Trang 2for conjugation with drugs, ligands, antibod-ies and genes, as well as functional groups of interest Excitation and relaxation of conduction band electrons in metallic nanoparticles results
in plasmon resonance – a unique phenomenon responsible for energy dissipation and optical effects An effective cellular communication, the invincible ability of nanometals provides immense possibility for their growth and devel-opment in biomedicine Nanoparticles have been synthesized, tested, used and modified over the years to function as agents for gene therapy, DNA sequencing, cancer detection, cellular tracking, targeted drug delivery and biomedical imaging
The versatile attitude of metallic nanoparticles attracts scientific research and clinical appli-cations under the stream of nanotechnology
(T able 1)
Aluminosilicate nanoparticles
Types, properties & synthesis of aluminosilicate nanoparticles
Types
Aluminosilicates are broadly classified into the following categories (F iguRe 1):
Orthosilicates with [AlO6]3- anions connected
by isolated (SiO4)4- clusters; for example, andalusite (Al2SiO5) and its polymorphs, kyanite and sillimanite;
Phyllosilicates with tetrahedral and octahedral layers in two dimensions; for example, kaolinite, smectite and illite;
Cyclosilicates with tetrahedral clusters of (Si3O7)6-, (Si4O12)8- or (Si6O18)12- arranged in
a cyclic manner; for example, bentonite (BaTi[Si3O9]) and beryl (Be3Al2[Si6O18])
Properties
Aluminosilicates are minerals consisting of alu-minium and silicon oxides Silicates are tetra-hedrally clustered polymers of (SiO4)4- anions
The positively charged ions of Al3+ can either substitute silica atoms in the silicate tetrahedra
or connect outside the anionic framework, to form aluminosilicates In nature, magma solid-ifies to form aluminosilicates such as feldspar (xAl[Al,Si]3O8, where x can be Na, K or Ca), mica, beryl or wollastonite In feldspar the Al3+
ion replaces the Si4+ cation of (SiO4)4-, leaving behind a negative charge on the 3D framework The positively charged ions neutralize this negative charge, for example, K+ in microcline (KAlSi3O8) and Na+ in albite (NaAlSi3O8), both K+ and Na+ in sanidine ([K,Na]AlSi3O8]4) and Ca2+ in Anorthite (Ca[AlSi2O8]) The weather plays its role to convert feldspar to clay kaolin (Al2Si2O5[OH]4) or montmorillonite ([Na,Ca]0.33[Al,Mg]2[Si4O10][OH]2.nH2O) Naturally occurring aluminosilicate nanopar-ticles exist as nanotubes called imogolites, or hollow 3–5 nm spherical allophanes Both these aluminosilicates have on identical chemical com-position (Al2SiO3[OH])4, but with different structures depending on the Al/Si ratio
Clay has negatively charged sites that can attract and hold positively charged particles and this is called ‘cation exchange capacity’; it
is the measure of how many negatively charged
sites are available on a nanoparticles surface
These exchange reactions are rapid, reversible and stoichiometric with respect to charge: 2{K + -Soil} + Ca 2+ → 2K + + Ca 2+ –(Soil)2
e quaTion 1
Aluminosilicate nanoparticles undergo ion exchange readily, that is, adsorbed cations can
be replaced by a large quantity of other com-peting ions, which superimpose their strength and resistance The layered sheet-like structure
of aluminosilicate nanoparticles provides addi-tional surface area as well as the ability to hold substances for targeted delivery
Kaolinites are 1:1 aluminium phyllosilicates having the chemical formula Al2Si2O5(OH)4 Clays such as kaolinite, dichite, nacrite and hal-loysite fall under this category They have SiO4 tetrahedrons and AlO4 octahedrons arranged in
Table 1 Examples of metallic nanoparticles used as drugs and diagnostic agents.
Metallic nanoparticles Element Use
Aluminosilicate nanoparticles Al, Si Faster blood clotting in open wounds
nanocrystalline structure
Trang 3a 2D hexagonal array This arrangement twists
the tetrahedral sheet, flattens the octahedral
sheet and compels the hexagonal arrangement
to distort in a ditrigonal manner
Smectites and illites are 2:1 bonded sheets of
aluminium phyllosilicates with an octahedral
sheet sandwiched between two tetrahedral sheets
(TOT) The space between two TOT sheets is
occupied by cations and/or water molecules The
arrangement can be designated as TOT (H2O/
cations) TOT Smectites have the chemical
for-mula: Na0.3 Al2(Si3.7Al0.3)O10(OH)2 and examples
include montmonrillonite, nantronite, saponite
and hectorite They have high cation exchange
capacity, a very large chemically active surface area
and an unusual tendency to hold water molecules
in the interlamellar surface Illites have the
chemi-cal formula K0.7Al2(Si3.3Al0.7)O10(OH)2 The
cat-ion exchange capacity of the illite group is midway
between that of kaolinite and smectite, but their
hydration capacity is low due to the replacement
of Na+ ions by K+ ions
Synthesis
Mesoporous aluminosilicate nanoparticles with
narrow size distribution (30–50 nm) are
syn-thesized by a hydrothermal method using
cet-yltrimethylammonium bromide as a template
and polyethylene glycol as a means to tailor the
nanoparticles [1] The aluminium salt,
alumi-num nitrate nonahydrate, catalyzes the
hydroly-sis of the silica precursor tetraethyl orthosilicate
The hydrolyzed species can be rapidly assembled
into mesostructured nanocomposites under the
direction of cationic micelles with the addition
of basic ammonia water The nonionic
polyeth-ylene glycol shields the formed nanoparticles
through hydrogen-bonding interactions, thereby
tuning the grain size distribution of mesoporous
nanoparticles A sol–gel route has been used to synthesize aluminosilicates, with varying alu-mina–silica ratios The process uses boehmite and tetraethyl orthosilicate as alumina and silica precursors, respectively [2]
Aluminosilicate nanoparticles in medicine Clay has been popular since the prehistoric era
in bath spas to preserve complexion; in ochres to cure wounds caused by serpents, to reduce the flow from the lachrymal ducts; against hemor-rhage, inflammation, gastrointestinal infections and kidney diseases; and even to make mum-mies Modern physicians use the same clay parti-cles at a nanoscale level in bandages, antibacterial ointments and pharmaceutical carriers
Kaolinites are known for wound healing
Kaolin clay has long been used for curing inju-ries, festering inflammations and healing wounds
From the 1950s kaolin has been an activating agent for a clotting test that doctors perform routinely The clay is predominantly rich in alu-minosilicate nanoparticles that have the ability
to reduce staunch bleeding by absorbing water;
resulting in quick blood clotting [101] The surgical dressings, impregnated with kaolin, are sold under the tradename ‘QuickClot®’ and are used to com-bat life-threatening hemorrhage on the com-battlefield
The presence of kaolin on the surface of nonwo-ven rayon gauge leads to enhanced transformation
of factor XII, factor XI and prekallikrein to their activated forms; this activation further initiates the coagulation cascade of hemostasis Chemists
at the University of California (Santa Barbara,
CA, USA), realized that the aluminosilicate nanoparticles could be used to halt severe nose bleeds The inorganic specks, which are derived from kaolin clay, when infused with a bandage,
Aluminiosilicates
Orthosilicates
Phyllosilicates
Cyclosilicates
Examples: andalusite, kyanite, sillimanite (used in ceramics, boiler furnaces and kiln linings).
Examples: kaolinite, smectite, illite (used in wound healing, burns, sepsis and inflammation)
Examples: bentonite, beryl (used as a laxative and anti-inflammatory agents)
Figure 1 Various types of aluminosilicates and their uses.
Trang 4trigger the body’s natural clotting process The bandage stops the bleeding immediately, when rolled up and inserted in the nose [102]
Smectites & illites possess antibacterial ability
Smectites are famous for their tendency to absorb the carcinogenic metabolite aflatoxin
B1, produced by the fungi Aspergillus flavus in
animal diet [3] The nanoparticles of smectites–
illites and reduced iron present in natural clay
have the potential to eliminate Escherichia coli
and even antibiotic-resistant bacteria such as
methicillin-resistant Staphylococcus aureus The
hydrated clay leaches into the bacterial cell mem-brane to increase bacterial iron and phosphorous levels and metabolic activity of the membrane
The regulatory proteins subsequently come into action to oxidize Fe2+ to Fe3+ and even produce hydroxyl radicals, which enter the cytoplasm and cause cell death [4]
The in vitro antibacterial activity of clay
min-erals has proven effective against Buruli ulcer
and b-lactamase E coli The mineral surfaces of
aluminosilicates in clay alter pH and oxidation states in bacterial membranes to control redox reactions, resulting in cell lysis [5] The layered metal hydroxides of clay behave as excellent pharmaceutical carriers The lamellar surfaces
of various layers can easily hybridize nano-medicines in their 2D structure For example, methotrextate – a folate antagonist anticancer drug – is unstable and also has a short plasma half-life The drug, when layered in Mg and Al hydroxides of clay, specifically suppresses growth
of human osteosarcoma cancer cells [6]
Bentonite: a versatile aluminisilicate
Bentonite is a chemically inert, absorbent alu-minium phyllosilicate consisting of montmoril-lonite Bentonite supports good digestion and acts as a laxative In the gastrointestinal tract of animals bentonite reduces bacterial mucolysis and inflammation The granular form of benton-ite is used under the commercial name ‘Wound-Stat™’, in battlefields for wound dressings It reduces pain associated with stings, burns and cuts, promotes detoxification and also shields against urushiol – the oil found in poison ivy
Nanobiomagnets
Types, properties & synthesis of nanobiomagnets
Types
The nanosized, biocompatible, paramag-netic iron oxides that serve as biomagnets are
magnetite (Fe3O4), maghemite (g-Fe2O3) and haematite (a-Fe2O3); of which magnetite, because of its biocompatibility, is very promis-ing Iron oxide nanoparticles (IONps) are avail-able in various dimensions and shapes such as nanorods, nanotubes, hollow fibers, rings and snowflakes
The iron oxide nanorods demonstrate higher incident photon-to-current conversion compared with nanospheres, which is further improved
by surface modification and doping with Zn The nanoparticle size imposes a huge impact on superparamagnetism and, in turn, their usage Generally, iron oxide nanoparticles ranging from
1 to 25 nm are highly efficient models Super-paramagnetic iron oxide nanoparticles are of particular interest in MRI; examples include:
AMI-227 (Sinerem, Combidex®) and SHU-55C – a 20 nm sized iron oxide nanoparticle coated with carbodextran It demonstrates excellent T2 relaxivities of 151.0 mmol/sec and has been used for lymph node and bone marrow imaging
OMP (Abdoscan®) and AMI-121 (Lumirem®, GastroMARK®) are 300 nm sized iron oxide nanoparticles (IONps) coated with silica Their oral administration finds utility as a gastrointestinal contrast
Properties
In magnetite, Fe3+ ions are placed at all tetrahe-dral sites, whereas both ferrous and ferric ions occupy octahedral sites of inverse spinel struc-ture Maghemite is the oxidized form of mag-netite having 56 ions in each unit cell, of which
32 are O2- ions, eight Fe3+ ions in tetrahedral sites and 16 Fe3+ in octahedral sites Magne-tite is a spin-polarized black crystal containing both Fe (II), Fe (III) and absorbs throughout the UV–vis–IR spectrum, while maghemite is
an insulator Both phases are ferrimagnetic Haematite, a-Fe2O3, has a 3D framework built
up of trigonally distorted octahedra FeO6, with oxygens in hexagonal closest-packing The tri-valent iron ions are closely packed between two oxygen layers This arrangement makes the structure neutral with no excess charge Haematite has antiferromagnetic properties and an absorption spectrum in the visible range between 295 and 600 nm The magnetic behav-ior of these oxides is due to their stereochem-istry that triggers internal superexchange com-petition between tetrahedral and octahedral
Trang 5sites At room temperature both magnetite
and maghemite are superpara magnetic, which
means an external magnetic field can easily
magnetize the nanoparticles
The IONps need to be superparamagnetic,
biocompatible and nontoxic to be useful for
molecular imaging purposes They also need to
bind to a range of metabolites The zero point
charge value of seven makes oxides stable only
in highly acidic or basic aqueous media This
drawback in their surface chemistry causes
con-siderable aggregation and precipitation in
solu-tion phase Also, low hole mobility,
electron-hole recombination and electon-trapping, and
oxygen-deficient iron sites yield poor
photocur-rent efficiency However, coating the particles
with silica, dextran, carbodextran, poloxamines
or poly(ethylene glycol) followed by their
bio-conjugation with various ligands gives them
both stability and specificity Magnetic
iron-oxides need high r1 and r2 relaxivities, as well
as surface engineering, to fine tune their size
and structure, before being used for in vivo
applications [7]
The ferromagnetic nanoparticles
magnetiza-tion fluctuates with temperature, fluctuamagnetiza-tions
are generally larger at higher temperatures and
smaller at lower temperatures When the time
between two magnetization fluctuations (Néel
relaxation time) is shorter than the time used to
measure the magnetization of the nanoparticles,
in the absence of external magnetic field, the
nanoparticles show an average zero
magnetiza-tion This is called superparamagnetism
Mate-rials having superparamagnetism have a high
saturation magnetization and zero coercivity
and remanence
The Néel relaxation time is highly
tempera-ture dependent, it fluctuates randomly by
ther-mal fluctuation at high enough temperatures
The thermal energy decreases at lower
tempera-tures and blocks the magnetic moments This
temperature is called the blocking temperature
It is a function of the particle size and increases
with increasing particle size Thus,
superpara-magnetism increases with the decrease in size
of the nanoparticle Below blocking
tempera-ture, the preferred direction of magnetization
of superparamagnetic material is lost in zero
magnetic fields When the temperature rises
above the blocking temperature, the
nanopar-ticles show no hysteresis With these fascinating
superparamagnetic properties, IONps find their
utility in ferrofluids, hyperthermia and MRI
contrast agents
Synthesis
Reverse micelle and precipitation are two com-monly used techniques for the synthesis of iron oxides [8] The simplest of all the methods to prepare IONps is the coprecipitation of a 2:1 stoichiometric mixture of Fe2+/Fe3+ salts in an aqueous medium of pH between 8 and 14 The magnetite forms black colored precipitates The overall reaction is written as:
Fe 2+ + 2Fe 3+ + 8OH - → Fe3O4 + 4H2O
e quaTion 2
The particle size depends on numerous fac-tors such as Fe3+/Fe2+ ratio, temperature, ionic strength, nature of salts, pH and addition of che-lating agents Generally, the nanoparticle size decreases with an increase in the pH, Fe3+/Fe2+
ratio and ionic strength of the medium
The aqueous iron salt solutions essentially form reverse micelles with the hydrophilic head towards the core of the micelle and the hydro-phobic tail directed outwards Reverse micelles solubilize large amounts of water, which can be controlled, for nanoparticle production A wide range of iron oxide nanoparticles can be syn-thesized by altering the nature and amount of surfactant, solvent and cosurfactant
They can also be synthesized using techniques such as sonochemistry, microwave irradiation and autogenic pressure reactor [9] A new method
to produce nanocrystals is glass crystallization [10] In total, 15–20 nm sized, monodisperse,
Fe3O4 nanoparticles are synthesized by decom-position of iron (II) acetate at 400°C IONps of desired size and dispersity are also synthesized by heating iron-oleic complex at 320°C in 1-octa-decene for 30 min Hydrothermal treatment of iron powder and iron chloride solution in urea solution for 20 h at 130–150°C yields iron rods
of nearly 80 nm
Photoelectrochemical applications of biomagnets
The chemistry of iron oxide nanoparticles can
be manipulated to have magnetic properties that find their importance in magnetic reso-nance imaging, biotechnology and effective hyperthermia (F iguRe 2)
Iron-oxide nanoparticles as hyperthermia
& MRI contrast agents
MRI is a noninvasive technique that combines the characteristics of high spatial resolution,
Trang 6nonionizing radiation and multiplanar tomog-raphies in cellular imaging Superparamagnetic iron oxide nanoparticles comprise a class of novel MRI contrast agents that are composed
of a ferrous iron (Fe2+) and ferric iron (Fe3+) core, and a layer of dextran or other polysaccharide coating [11] The iron nanoparticles have a very large magnetic moment, which leads to local magnetic field inhomogeneity Consequently, they serve to enhance the image contrast and, thus, improve the sensitivity and specificity of MRI in mapping information from tissues [12]
In vivo, nonspecific superparamagnetic iron
oxide nanoparticles are mainly captured by the reticuloendothelial system, and they are more suitable for liver, spleen and lymph node imaging [13] Because of their long plasma half-life, super-paramagnetic iron oxides are also used as blood pool agents in magnetic resonance angiography
Haematite nanoparticles, 1.8 nm in size, when coated with polysaccharides such as chi-tosan and alginate, respond superparamagneti-cally with very low coercivity These nanopar-ticles can either be converted to magnetite by reduction or used directly for imaging [14] The intensity of magnetic field of iron-based nanoparticles, having a layer of bis-carboxyl-ter-minated poly(ethylene glycol) on them, induces more effective hyperthermia than uncoated iron particles They are far better MRI contrast
agents and provide a focused approach for in vivo
applications and cancer therapy [15] Iron oxide nanoparticles manipulated with Herceptin® –
an antibody present in breast cancer cells – or chlorotoxin – a peptide that binds MMP-2 in
gliomas, show enhanced in vivo tumor-targeting
properties [16] Mammary tumors contain over-expressed levels of urokinase-type plasminogen activator Amino-terminal fragment conjugated IONps can effectively bind the over expressed
receptors in breast cancer tissues and help in vivo
imaging [17] The nanocomposites of maghemite,
such as those with bentonite and raffinose-mod-ified trypsin, are used as MRI contrast agents for the gastrointestinal tract and magnetic carriers for trypsin immobilization, respectively [18] Flu-orescence and magnetism can be uniquely com-bined over maghemite nanoparticles Congo-red
or rhodamine dyes hybridize with g-Fe2O3 to
serve as biomarkers for in vivo Alzheimer’s
dis-ease diagnosis [19] Ferrite nanoparticles ranging from 20 to 200 nm in diameter are being used for biosensing and as contrast agents for MRI, when attached with europium these spheres can emit fluorescent radiations at 618 nm to help detect cancer [20]
Nanobiomagnets in biotechnology
Biotechnology can rely on the magnetic powers
of IONps to separate specific proteins from a group of biomolecules For example, dopamine grafted IONps can be used for protein separa-tion The bidentate enediol ligands of the dopa-mine molecule tightly bind with unsaturated iron sites The nanostructures so produced enhance specificity for protein separation and provide tremendous stability to heating and high salt concentrations In the same manner, mag-netic nanoparticles are ideal candidates for gene detection In the diagnosis of diseases involving genetic expression, the separation of rare DNA/ mRNA targets with single-base mismatches
in a mixture of various bio complexes is criti-cally important Genomagnetic nano capturers have been formulated using IONps to detect DNA/RNA molecules with one single-base dif-ference Nanobiomagnets can transfer drugs into the body and are held at the target site by
an external magnet The purpose of this is to concentrate the drug at the tumor site for long enough for it to be absorbed and release the drug
on demand The control of drug delivery using biomagnets can reduce the dosage by 60–75%, thus enhancing drug efficacy while decreasing
Nanobiomagnets
Magnetite
Maghemite
Haematite
Used in hyperthermia,
in biotechnology as genomagnetic capturer, as MRI contrast agents and to magnetically focus the drug at tumor sites
Photoelectrochemical applications
Figure 2 Nanobiomagnets and their photochemical applications.
Trang 7unwanted systemic uptake This mechanism can
find its utility in control of insulin-dependent
diabetes Recent studies report that the iron
oxide nanoparticles can adhere to red blood cell’s
surface for nearly 4 months This can help to
release drugs slowly into the body and can lead
to controlled treatment of many immunogenic
diseases
Q-dots
Types, properties & synthesis of Q-dots
Types
Q-dots are tiny particles, traditionally
chal-cogenides (selenides or sulfides) of metal such
as cadmium or zinc (CdSe/ZnS) ranging
from 2 to 10 nm The electrons and holes of
the semiconductor cores being confined to a
point significantly modifies the energy
spec-trum of the carriers Q-dots have a metallic
core made of semiconductors, noble metals,
and magnetic transition metals, shielded by a
shell Depending on the variation in the
con-stituents of the core, Q-dots are classified into
various groups:
Group II–IV series Q-dots contains ZnS,
ZnSe, CdSe and CdTe cores;
Group III–V series Q-dots have InAs, InP,
GaAs and GaN cores;
Group IV-VI series Q-dots have PbTe, SnTe,
SnS and SnS2 cores
Q-dots are also classified as Type-I, Reverse
Type-I and Type-II:
Type-I Q-dots have a core that simultaneously
traps electrons and holes giving rise to
contravariant band layout so that both the conduction and valence band edges of the core lie within the bandgap of the shell; for example, CdSe/CdS, CdSe/ZnS and InAs/CdSe;
For reverse Type-I Q-dots, the bandgap of the core is wider than the shell, and the conduc-tion and valence band edges of the shell lie in the core; for example, CdS/HgS, CdS/CdSe and ZnSe/CdSe;
Type-II Q-dots have one type of charge carrier
in the core while the shell carries the other type It maintains covariant band layout in which the valence and conduction band edge are either lower or higher than the band edges
of the shell; for example, Type-II Q-dots that attract holes are GaSb/GaAs and Ge/Si, and those that attract electrons are InP/InGaP and InP/GaAs
Properties
Q-dots are basically made of three parts – a core, a shell and the outer coating (F iguRe 3) The core region, when excited by a photon, triggers its electron in the semiconductor band gap, leaving behind a positive hole in the lower energy band An increase in excitation increases the absorption in the band gap giving rise to broad absorption spectrum Since the energy gap between higher and lower energy bands is responsible for emission energy, and the energy gap is low, the emission spectrum is narrow
The shell covers the surface defects of the elec-tron–hole nanocore, and thus protects it from oxidation, fluorescence and chemical reactions
The shells having large energy gaps increase the quantum yield and enhance photostability A
Q-dot core (CdSe)
Shell (ZnS)
Cap (disulfide
bridge, silane)
S (protein/DNA)Biomolecule
Biological applications
Cellular and assay labeling
High-resolution cell imaging
Q-dot-FRET biosensing
Figure 3 A biofunctional quantum dot and its biological applications
FRET: Fluorescence resonance energy transfer; Q-dot: Quantum dot; S: Sulfide bridge.
Trang 8coating of functional ligands over the Q-dot shell improves their solubility in polar solvents and also labels them The mono or dithiol dihy-drolipoic acid ligands improve stability for over 1–2 years; phospholipids induces stability over a wide pH range while thiolated peptides or poly histidine residues provide both dispersion and bio-functionalization
The electronic properties of Q-dots are inter-mediate between those of bulk semiconductors and discrete molecules The most apparent of these is the emission of photons under excitation, which are visible to the human eye as light The wavelength of these photon emissions depends
on their size The smaller the dot, the closer it is
to the blue end of the spectrum and the larger the dot, the closer to the red end The charac-teristics of Q-dots that attract the attention of biomedicine are brightness, time resolved imag-ing because of 20 s lifetime of fluorescence, and the ability to image many colors simultaneously without overlapping, due to narrow fluorescence emission Moreover, Q-dots require a mini-mum amount of energy to induce fluorescence, resulting in high quantum yields Their core-shell structure makes them highly stable against photobleaching
Synthesis
The binary semiconductor nanocrystals such as cadmium selenide, cadmium sulfide, indium arsenide and indium phosphide could
be synthesized by fabrication, colloidal syn-thesis or as viral assembly Bulk quantities of semiconductor dots are produced by colloidal synthesis based on a three-component system composed of precursors, organic surfactants and solvents The high temperature turns the reaction medium to monomers [21] Fabrica-tion produces 5–50 nm sized dots, defined by lithographically patterned gate electrodes, or by etching on 2D electron gases in semiconductor heterostructures [22] The biocomposite struc-tures of Q-dots could be genetically engineered using bacteriophage viruses (TMV, M13 or Fd) [23] Subjecting the organometallic precursors (CdO, Cd-acetate) and solvent-ligand (trioctyl phosphine–tri octyl phosphine oxide) mixture
to high temperatures yields CdSe Q-dots with high crystalline cores
The utility of Q-dots in medicine The initial Q-dot bioconjugate was reported
in 1998 Over the past decade, the study of Q-dots has extended from high-resolution
cellular imaging to labeling, tumor targeting, and diagnostics
Q-dots in cellular & assay labeling
When introduced in cells, Q-dots found applica-tions in cell tracking, immunoassays, determin-ing the metastatic potential of cells and unleash-ing various cellular and metabolic processes The labeling of cells and assays with Q-dots is
an initial step of imaging processes and can be achieved by extracellular or intracellular modes The proteins as well as receptors associated with membranes help in extracellular labeling
of Q-dots to understand biological pathways such as signal transduction, chemotaxis, cel-lular organization and diffusion behavior of metabolites Studies have been successfully carried out with biotinylated-coated dots and glycosyl–phosphatidyl–inositol conjugated avi-din Q-dots to understand the diffusive behavior
of the plasma membrane [24]
To demonstrate intracellular labeling, cells can be microinjected or incubated with Q-dots via nonspecific endocytosis The peptide-medi-ated intracellular delivery of Q-dots allows pas-sive intake of biomolecules, such as cytokeratin, mortalin, microtubules, liposomes and oligonu-cleotides, into the cells The streptavidin–biotin complex links easily, through covalent bonding
to the Q-dot surface to control intracellular delivery
The difference between invasive and non-invasive cancer cell lines can be demonstrated
by in vitro cell motility assay based on the
phago-kinetic uptake of Q-dots The cell lines move across the homogeneous layer of Q-dots and leave a fluorescent-free trail On calculating the ratio of trail-to-cell area, the tumor invasiveness can be easily distinguished
Q-dots coated with DNA serve as probes for the detection of multiallele DNA and human metaphase chromosomes They also act as
spe-cific DNA labels for highly sensitive in situ
hybridizations [25] Multiple toxin ana lysis in immunoassays and marking Her2 breast cancer cells has been possible by conjugating Q-dots with antibodies [26] Q-dots have also been used
to diffuse glycine receptors in neurons and in near-infrared emission identification of lymph nodes during live animal surgery [27]
Recently, CdTe Q-dots have been reported
to control the nerve cells Light energy excites electrons in the Q-dot, which causes the immediate environment to become negatively charged This cause the ion channels to open
Trang 9in the cancerous tissue, allowing the
thorough-fare of ions in and out of the cells The ion
channel openings generate action potential over
nerve cells, which in turn can be controlled by
external voltage on Q-dots to depolarize the
unwanted cells [28]
High-resolution cell imaging with the help
of Q-dots
Q-dots have the ability to overcome the
limi-tations of fluorescence imaging of live tissues,
which is greatly hindered by the poor
trans-mission of visible light Q-dots act as the
inor-ganic fluorophore for intra-operative detection
of tumors using fluorescence spectroscopy as
they are 20-times brighter and 100-times more
stable than the traditional fluorescent
report-ers [29] The improved photostability of Q-dots
allows the acquisition of many consecutive focal
plane images that can be reconstructed into a
high-resolution 3D image The extraordinary
stability makes them a probe to track cells or
molecules over extended periods of time The
ability to image single-cell migration in
real-time renders their importance in
embryogen-esis, cancer metastasis, stem-cell therapeutics,
lymphocyte immunology and in vitro imaging
of prelabeled cells [30] Fluorescent Q-dots can
be tagged to antibodies that target cancerous
cells or cells infected with tuberculosis or HIV
[31], and could also be used to diagnose malaria
by making them target the protein that forms a
mesh in the blood cell’s inner membrane The
shape of this protein network changes when
cells are infected with malaria, so scientists are
able to spot malaria infection from the shape
produced by the dots [32] Q-dots have earned
success in sentinel lymph node biopsy, a
tech-nique that locates the first draining lymph node
at the cancer site The background tissue
auto-fluorescence is an avid limitation of blue dye and
radioisotopes used in biopsy, which is overcome
by Q-dots emitting at the near-IR range This
allows surgeons to undertake biopsy with high
accuracy and minimum invasiveness
Q-dot-fluorescence resonance energy transfer
biosensors
A fluorescence resonance energy transfer
(FRET) is an energy transfer between two
chro-mophores through dipole–dipole interactions
The process of energy transfer enjoys an inverse
relationship to the sixth power of the distance
between donor and acceptor molecules FRET
can wonderfully detect molecular interactions
and conformations in biological systems Q-dots can transfer their energy to quencher analytes through FRET, thus minimizing the fluores-cence from the Q-dot donor Gold rods read-ily quench the fluorescence from the Q-dots
This exciting property of FRET between Q-dots and the surface of gold nanoparticles helps to explore many DNA properties [33] The friend-ship between FRET technology and Q-dots can reduce background signal due to time-gating and increase the possibility of measuring long distances
FRET-based Q-dots biosensors have been
developed to detect Aspergillus amstelodami
The Q-dots conjugated to IgG antibodies transfer their energy to quencher-labeled ana-lytes through FRET The high-affinity target analytes replace the quencher analytes during detection to increase Q-dot fluorescence sig-nal The sandwich immunoassay then detects
Aspergillus, as low as 103 spores/ml, in 5 min
The idea can be further exploited to detect other biological threats [34] Multiple colored Q-dots can tag various antibodies uniquely Recently, researchers demonstrated a novel idea to multi-plex the utility of Q-dot biosensors, by applying simultaneous FRET to five different Q-dots on terbium complex with emission maxima at 529,
565, 604, 653 and 712 nm [35] CdSe-ZnS core-shell Q-dots coated with dihydrolipoic acid and conjugated with human phosphoinositide-dependent protein kinase-1 have been designed to identify selective inhibi-tors of protein kinases The response of this bio-sensor is tested in molecular dyad incorporating
an ATP ligand and a chromophore The organic dye allows nonspecific adsorption on the surface
of nanoparticles promoting FRET from Q-dot
to quencher dye The assay demands study of new strategies to prevent energy adsorption on the nanoparticle donor surface [36]
Cochleates
Types, properties & synthesis of cochleates
Types
Cochleates are multilayered delivery vehicles made of alternating layers of divalent counter ions (Ca2+/Zn2+) and bridging phospholipid bilayers, all rolled up in a spiral [37] They are made up of three constituents: the lipid bilay-ers, the cations and the agent to be delivered;
on varying one or more of these constituents, various permutation combinations are possible,
as shown in b ox 1
Trang 10Cochleates are rod-like, rigid, internally hydro-phobic sheets made from small unilamellar lipo-somes condensed by bivalent cations The positive charge on cations such as Zn2+, Ca2+, Mg2+ and
Ba2+ interacts with negatively charged lipid to con-dense it and rolling further makes them resistant
to their immediate environment The high ten-sion at the bilayer edges of cochleates is the driv-ing force of cochleate’s interaction with the tissue membrane [38] The cell membranes fuse with the lipid bilayer structure of cochleates, which unfolds
to release the internal contents into cells Another hypothesis put forward is the idea of phagocytosis for nanocochleates’ delivery The phosphatidylser-ine receptors are common between the liposomal membranes of macrophages as well as those of cochleates When in close proximity, the liposome membrane and the outer cochleate layer fuse to release the drugs into cell cytoplasm
The alternating lipid layers entrap the drug molecules without chemically bonding to it and potentially protect it from digestive enzymes in the stomach Encochleation is a medium to extend the shelf-life of drugs because the cochle-ate cores are resistant to wcochle-ater and oxygen, two components that act as leading agents of drug decomposition and degradation
Synthesis
Cochleates can be produced in submicron size using methods known as hydrogel-isolated cochleation, trapping, binary aqueous–aqueous emulsion, liposome before cochleates dialysis, direct calcium dialysis, or simply by increasing the ratio of multivalent cationic peptides over nega-tively charged liposomes The hydrogel method immerses unilamellar liposomes loaded with drug
in two sets of immiscible polymers The polymer miscibility results in a two-phase aqueous system, which is crosslinked by a cation salt The tiny
cochleates so formed are washed and then sus-pended in a buffer The trapping method involves dropwise addition of calcium salt and water phase
to the formative layer of phosphatidylserine lipo-somes The binary aqueous–aqueous emulsion method injects the primary dextran–liposome phase into a secondary non-miscible polyeth-ylene glycol polymer The divalent cations are then diffused from one phase to another forming cochleates, less than 100 nm in size
The liposome before cochleates dialysis method suspends a detergent–lipid mixture in
a two-phase polymer system The mixture is dia-lyzed with a buffer to form protein–lipid vesi-cles The cochleate precipitates from the vesicles
by addition of calcium ions Large needle-shaped cochleates are formed by the direct calcium dial-ysis method, in which lipid detergent mixture is dialyzed against CaCl2 solution [39]
The cochleate technology for nanomedicine
Cochleate means spiral shell In 1975 Papah-adjopoulos and Wilschut discovered cigar-like nanocochleates nearly 500 nm in size Since then, cochleates have been used to formulate a variety of biologically active molecules, mediate effective oral drug bioavailability and reduce toxicity There is a budding interest of scientists
to explore cochleate efficacy in gene delivery
Effective drug delivery by cochleates
Cochleate technology is a new means of over-coming the poor oral absorption of drugs such as amphotericin B and to facilitate the bioral drug delivery of cochleate-administered oral doses of amphotericin B, ranging from 0 to 40 mg/kg of body weight/day fortnightly in a murine model of systemic aspergillosis This leads to a reduction of more than two logs of colony counts in hepatic, pulmonary and renal organs [40] Cochleates increase the efficacy of antibacterial drugs such as clofazimine used against tuberculosis To protect mice from lethal acute graft-versus-host disease the immunosuppressive, water-insoluble com-pound 3-(2-ethylphenyl)-5-(3-methoxyphenyl)-1H-1,2,4-triazole was subcutaneously adminis-tered through an oily vehicle The oral admin-istration of 10 mg/kg of this compound after encochleation reduced lethality, and increased the survival rate to 100%, whereas the control with empty nanocochleates was inactive [41] Cochleates serve as delivery vehicles for anti-inflammatory drugs such as naproxen, ibupro-fen and acetaminophen Macrophages use the
Box 1 The various constituents of a cochleate.
Zn 2+ , Ca 2+ , Mg 2+ , Ba 2+ Phosphotidylserine
phosphatidic acid Phosphotidylinosotol Phosphotidyl Glycerol Phosphotidylcholine Phosphotidylethanolamine Diphosphotidylglycerol Dioleoyl phosphatidic acid Distearoyl phosphatidyl serine Dipalmitoylphosphatidylglycerol
Protein Peptide Polynucleotide Antiviral agent Anaesthetic agent Anticancer agent Immunosuppressant Anti-inflammatory agent Tranquilizer
Nutritional supplement Vitamins or
Vasodilator agent