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List of methods used for inspecting and confirming APTES monolayer <br>
<br>
and/or multilayer formation on oxide surfaces. After the APTES <br>
<br>
grafting step, a post-deposition process (i.e.,<br>
<br>
rinsing and baking) is usually performed to remove the non-covalently attached silane molecules without affecting the covalently-bonded silane molecules.<br>
<br>
As the result of the condensation reaction between silanol groups <br>
<br>
on the APTES molecule and -OH groups on a pre-treated oxide surface or between neighboring hydrolyzed APTES molecules, the formation of siloxane bonds occurs (Figure <br>
<br>
1A). Due to this strong covalent bond, bioreceptors can be chemically attached to the silane layer via charge-to-charge interactions or bifunctional crosslinkers without disrupting the silane structure 16,17.<br>
<br>
Among all organosilanes, APTES is the most used silane <br>
<br>
molecule to functionalize oxide surfaces 5,6,7,8,9,10,11.<br>
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<br>
Saini et al. used YES CVD to deposit monolayers of APTES onto oxide surfaces that were cleaned and hydroxylated with piranha <br>
<br>
solution without using hydration, dehydration, or N2 purges step .<br>
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Another piece of research utilizing YES CVD used plasma-activated oxide surfaces,<br>
<br>
but instead of dehydrating them, they were hydrated with 500 <br>
<br>
L of water . After deposition, the non-covalently attached APTES molecules were removed using 6% acetic acid, effectively removing the <br>
<br>
multilayer and leaving the surface with an APTES <br>
<br>
monolayer, as proved by AFM and XPS surface analysis.<br>
<br>
Exposure of the oxide surfaces to a dry-cleaning process, such as oxygen plasma and UV-ozone cleaning for a certain period (5–30 min), has also been used for surface cleaning and activation 70,80.<br>
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The chemical reaction between the APTES and oxide nanoparticles’ <br>
<br>
(NP) surface is similar to the reaction with plain oxide surfaces.<br>
<br>
According to the molecular structure, an APTES monolayer should have an average chain length of ∼5–10 <br>
<br>
Å 5,44,64,65, a 0.5–0.8 nm thickness 44,53,66,67,68, an average density of 2.1–4.2 molecules per <br>
<br>
nm2 64,69 and a silanol group density of 5 per nm2 43,50. In an ideal <br>
<br>
situation, the attachment of the bioreceptors on an oxide surface via silane <br>
<br>
molecules should start with the hydrolysis of the <br>
<br>
silane molecules followed by condensation without the formation of additional <br>
<br>
bonds or different reaction routes.<br>
<br>
The amine group of the silane molecule is used <br>
<br>
to attach bioreceptors to the surface (Figure <br>
<br>
1A); however, it can also form a weak hydrogen bond with the surface silanol group or with the silanol group of other silane molecules (Figure 1B).<br>
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In addition, the silanol groups of different <br>
<br>
silane molecules can interact with each other, thereby forming polymeric structures (Figure 1B(5–7)).<br>
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This procedure was repeated three times and 2% APTES in 100 ml of methanol was used to cover the NPs surfaces with <br>
<br>
silane molecules. They simply introduced 0.3 mL of APTES via an argon carrier gas (27 sccm) to a deposition chamber containing the oxide surfaces, which were hydroxylated with oxygen plasma.<br>
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Amino-functionalized organic films were prepared by self-assembling <br>
<br>
3-aminopropyltriethoxysilane (APTES) on silicon wafers in either <br>
<br>
anhydrous toluene or phosphate-buffered saline (PBS) for varied deposition times.<br>
<br>
APTES films exhibit varying thicknesses and structural <br>
<br>
characteristics, with thicker films observed in anhydrous <br>
<br>
toluene (ranging from 10 Å to 144 Å) compared to thinner <br>
<br>
films produced in phosphate-buffered saline (8 Å to 13 Å).<br>
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The investigation reveals that the formation and structure <br>
<br>
of APTES films on silicon substrates are significantly influenced by deposition conditions, including the choice of solvent and deposition time.<br>
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A condensation reaction can happen between two <br>
<br>
APTES molecules and/or between the oxide surface and an APTES <br>
<br>
molecule . In the absence of water or if the water content is too low, the silanol formation could <br>
<br>
be incomplete 10,44,52,53. The water content determines the number of hydrolyzed groups <br>
<br>
during the hydrolysis step in the APTES <br>
<br>
molecule (hydrolysis of one, two, or three ethoxy groups) .<br>
<br>
The quality of the silane surface, including its morphology, strongly depends on the initial <br>
<br>
hydrolysis step, making it very critical . (A) Schematic representation of an APTES molecule, activation, and reaction with the hydroxylated oxide surface in an ideal situation. APTES has three functional reactive <br>
<br>
ethoxy groups and one amine group per one silane molecule (Figure 1).<br>
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<br>
Most NPs coated with APTES are used in applications such as <br>
<br>
for chemical sensors, cosmetic products (e.g., UV scattering effect in sun cream), drug delivery, magnetic resonance imaging,<br>
<br>
and bioimaging . Table 2 summarizes all the referenced solution-phase and vapor-phase processes and their monolayer <br>
<br>
characterization results. After deposition, the argon and APTES were removed, and the <br>
<br>
pressure was reduced to 75 × 10−7 Torr. Like YES CVD, there is some <br>
<br>
variation in the details of MLD processes, for example, they can be undertaken with or without high <br>
<br>
temperatures, a substrate dehydration, and nitrogen purges.<br>
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This preceded the ten minute deposition of 500 L of APTES at 500 Torr,<br>
<br>
after which several nitrogen purges were used to remove excess silane.<br>
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The desorption of APTES molecules from the surface was negligible in buffer solutions such as PBS compared to water.<br>
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<br>
Dissolving APTES in methanol with some water traces resulted <br>
<br>
in forming an APTES layer with 8.1 Å thickness based on spectroscopic ellipsometry measurements, which is close to the thickness of a monolayer <br>
<br>
. As previously mentioned, the amount of water in a solution and <br>
<br>
native -OH groups on an oxide substrate is critical in controlling the level of polymerization of silane molecules.<br>
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XPS studies revealed that, compared to a deposition in toluene, the <br>
<br>
presence of N and C was three and two times greater, respectively, indicating more than one monolayer of APTES.<br>
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Studies conducted by Pasternack et al. confirmed that lower APTES concentrations (0.1%) dissolved in toluene could create densely <br>
<br>
packed propyl chains on SiO2 when the reaction was carried out at 70 °C for less than 1 <br>
<br>
h. The deposition of APTES, in this case, was conducted on silicon wafers in a closed container (i.e.,<br>
<br>
a toluene solution, and an APTES concentration range of 1–33%) backfilled with N2 gas.<br>
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Amino-terminated organic layers were deposited onto silicon (Si) wafers and PS <br>
<br>
layers via 3-aminopropyltriethoxysilane (APTES) prepared in freshly hydrolysis solution. <br>
<br>
XPS results revealed the presence of Fe, O,<br>
<br>
C, and N, consistent with the development of interlinking silane molecules on a <br>
<br>
NP’s surface. Karade et al. conducted a ninhydrin colorimetric assay to monitor the monolayer <br>
<br>
formation through the quantification of the amine groups on the NPs .<br>
<br>
Additionally, APTES grafting on Fe2O3 magnetic NPs can prevent oxidative damage and play a role in the hydrophilicity or hydrophobicity of the NP surface, which is important for further drug conjugation through terminal amine groups.<br>
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Moreover, the experimental grafting density was very close to the <br>
<br>
theoretically predicted APTES monolayer density.<br>
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The study revealed that the equilibrium adsorption capacity was <br>
<br>
301.2 mg/g for the highest grafting density during <br>
<br>
incubations at 70 °C using 2% APTES. The formation of <br>
<br>
more APTES monolayers on nanoparticles was proposed by Liu <br>
<br>
et al., where good quality silane layers were formed when the reaction was carried <br>
<br>
out at an elevated temperature using methanol/toluene mixed solvents .<br>
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References: <br>
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