Essay On Kinetics of Au Colloid Monolayer

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Experiment: 2NANO2 Kinetics of Au Colloid Monolayer Self-Assembly

AIM:

The objective of performing this lab experiment was to investigate the kinetic factors involved in constructing 2-dimensional arrays of metal nanoparticles on derivitised glass surfaces using UV-Visible spectroscopy. On establishing the time dependence of surface assembly, the equilibrium constant for adsorption of the Au particles onto the surfaces was obtained by fitting a suitable isotherm to UV-Visible data as a function of particle concentration.

INTRODUCTION:

For many important materials, their properties are determined by only a very small percentage of atoms that are found on the surface. The inorganic self-assembled monolayer surface chemistry controls the properties of bioimplants, semiconductor devices, heterogeneous catalysts and many other materials. The mechanical properties and superior stability of inorganic monolayers has advanced the knowledge in the field of surface chemistry. Surface features that are functionally significant can be obtained on the scale of atoms, molecules or larger supramolecular assemblies or particulate. With a detailed knowledge of understanding of surface chemistry, chemists are determined to create or improve surface features for new applications [3]. It provides an opportunity to control the structure of the film at molecular level as well as creating tailored surface properties by incorporating different organic functional groups in the adsorbate molecules. Organic self-assembled monolayers have been potentially utilized in molecular electronic demonstrations, sub-micrometer lithographic patterning schemes and sensors. However, undesirable features such as instability in thermal conditions, desorption in organic solvents and oxidation on exposure to air limit the practical application of the organic self-assembled monolayers [2].

The surface plasmon for colloidal Au is located in a position between 500 and 600 nm, depending on the size and shape of particles, refractive index of solvent or absorbate, and the distance between particles. The maximum wavelength () for hydrosols of spherical, 12-nm-diameter Au is about 520 nm. The peak width provides information on the polydisperity of the colloidal solution, while the broader peaks show a greater particle size distribution. The surface plasmon absorbance also has a sensitivity to the spacing of colloidal particles when the nanoparticles flocculate, a second absorbance feature starts to grow at between 650nm and 750 nm, resulting in a deep blue solution. The appearance of this colour is an indication of aggregation in Au hydrosols [4].

This lab experiment involves kinetics investigation of Au nanoparticle self-assembly using glass microscope slides that are coated with a bi-functional organosilane. A reaction between the organosilane and the hydroxyl groups of the substrate occurs when clean glass slides are exposed to a solution of 1% 3-aminopropyltrimethoxysilane (APTMS) to form a siloxane bond. Au nanoparticles are formed when the salinized surfaces are exposed to Au colloid after rinsing.

EXPERIMENTAL PROCEDURE:

Refer to Manual for 2NANO2 “Kinetics of Au Colloid Monolayer Self-Assembly” pages 25-27.

RESULTS AND DISCUSSION:

It was not possible to obtain our own accurate results for this experiment because the solution that was prepared could not yield the anticipated results, as it was affected due to waiting as another group was using the UV spectroscopy. The results used in the write-up of this lab report were given by the lecturer.
The graph below shows the graph obtained from the UV-Visible spectrometer when a solution of diluted Au colloids was measured in a wavelength range of 350-800 nm.

Figure 1: Graph of Wavelength (nm) versus Absorbance for diluted Au colloids

The graph resembles a typical absorbance graph for 12 nm Au when APTMS derivatized slide was immersed in the solution.
The optical spectra obtained for wavelength versus absorbance at given times of 2, 4, 8, 10, 15, 20, 25, 30, 40, 50 and 60 minutes is shown in the figure 2 below.

The colloid particles of gold possess negative charges and bind on the coated surfaces by electrostatic forces with the molecules of organosilane. The deep red colour of the gold particles facilitates monitoring of formation of surface on the transparent substrates by measurement of optical spectra. From figure 2, it is observed that absorbance and peak absorbance increases as time increases.

Colloidal Au nanoparticles are suitable for use in building blocks because they possess several properties which can be utilized in surface modification. First, monodisperse colloidal Au solutions with average particle diameter of between 3 nm and 150 nm can easily be prepared, allowing for a variety of repeating feature sizes. Second, metal nanoparticles possess surface reactivity that is amenable to immobilization on surfaces that are chemically functionalized. Thirdly, colloid-based surfaces can be readily prepared using simple wet-chemical methods, eliminating the need for expensive, sophisticated equipment. Finally, aqueous solutions of Au nanoparticles enable following of surface formation by a UV-vis spectrophotometer or by eye because the nanoparticles are an intense red colour due to light absorption by free oscillations of free electrons or surface plasmons [1].

ANALYSIS OF RESULTS:

QUESTION 1. Write a proposed mechanism for the reaction of the APTMS with the silanol groups on the glass surface.
APTMS
Alkoxysilane groups of APTMS is hydrolyzed by addition of water to form silanol groups to facilitate salinization reaction. This results in the formation of n-HA-APTS with amines on the surface.
QUESTION 2. The Au nanoparticles imaged in Figure 2 are clearly spherical and monodispersed. Calculate the concentration in moles/L of Au particles formed from a 100 mL solution of 1 mMHAuCl4 assuming (i) all the particles are 12 nm diameter spheres; (ii) all the particles have the same size (iii) all Au3+is reduced to colloidal gold; and (iv) the density of colloidal Au is that of the bulk material (18 g/cm3).
(i)[(0.0339g/196.97g) /100Ml] = 0.00172 ML-1
(ii)0.00172 ML-1 (same as above)
(iii)If all Au3+ is reduced to colloidal gold, the concentration will be reduced to zero.
(iv)Concentration = [(18g/196.97g)100]/1000 = 0.009138 ML-1

QUESTION 3. Calculate the probability that a Au particle will stick to the surface using a non-linear least squares fit of Abs = kt1/2 to your data and Equations 1 to 3.

Slope of the graph = 0.003 = k
Converting k to units of Γ (particles),

Calculating the diffusion coefficient
Where:
k = Boltzman constant (1.38g )
T = temperature (K)
= viscosity (0.010 g )
r = particle radius (cm)

Calculating the colloid coverage,
= 6.02 )/

Calculating the colloid concentration, C

[(0.0339g/196.97g) /100Ml] = 0.00172 ML-1
=1.72
C = 1.0358 particles/

Calculating the probability (p) that a particle reaching the surface will absorb

From the equation ,

p = = = 0.0004

Sources Of Errors/ Uncertainities In This Experiment

The glass slides can interfere with the samples if not proprly handled or well rinsed. Traces of silane on the slides inhibits deposition of gold. This will affect the accuracy of the results obtained. To avoid errors resulting from this, the glass slides have to be cleaned and functionalized properly.
Also, delays in measuring UV-Vis spectra of the Au colloid can give poor results as the solution integrity diminishes.

CONCLUSION

Gold colloidal monolayers can simply be preared on polymer-coated substrates by self-assembly. The method is very simple and it essentially consists of immersion of a solid substrate into solutions of surface hydroxyl group, organosilane polymer, and immobilization of polymer. Gold particles are powerfully bound to the suface of polymer fucntional groups by covalent bonds. The functional groups provide active sites where Au is attracted. When the substrate become optically transparent throgh the visible light, UV-vis can asses both spacing and particle coverage. TEM on coated grids derivatized with a film of organosilane and colloidal gold indicate that there is no particle aggregation on the surface. The particles are also confined within a single layer.

References

[1]Grabar, Katherine C., et al. "Preparation and Characterization of Au Colloid Monolayers." Analytical Chemistry (1994): 735-743 .
[2]Hayat, M. A. "Colloidal Gold: Principles, Methods and Applications." (1989).
[3]Keating, Christine D., et al. "Kinetics and Thermodynamics of Au Colloid Monolayer." Journal of Chemical Education (1999): 949-955.
[4]Tang, Zhiyong, et al. "Self-Assembled Monolayer of Polyoxometalate on Gold." Langmuir (2000): 4946-4952.