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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.
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 . 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 .
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 .
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.
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 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 .
ANALYSIS OF RESULTS
QUESTION 1. Write a proposed mechanism for the reaction of the APTMS with the silanol groups on the glass surface.
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
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
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.
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 through 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.
Grabar, Katherine C., et al. "Preparation and Characterization of Au Colloid Monolayers." Analytical Chemistry (1994): 735-743 .
Hayat, M. A. "Colloidal Gold: Principles, Methods and Applications." (1989).
Keating, Christine D., et al. "Kinetics and Thermodynamics of Au Colloid Monolayer." Journal of Chemical Education (1999): 949-955.
Tang, Zhiyong, et al. "Self-Assembled Monolayer of Polyoxometalate on Gold." Langmuir (2000): 4946-4952.
Ambient desorption ionization mass spectrometry allows objects to be analyzed directly in the laboratory or natural environment, analyte desorption and ionization accompany each other. Ambient desorption does not require sample preparation and if it does it is just a little. It allows the work to be simple and flow easily. It also allows for the delivery of easy use MS analyses. There have been many demonstrations of permutations for analyte ionization and desorption. The steps of desorption include dissolution in ricocheting droplets, momentum transfer and thermal desorption ,are combined with the steps of ionization which include photoionization, the atmospheric pressure chemical ionization and ESI (.A.Kofmarcher, 2009). Proliferation techniques have used possible desorption combination and ionization components to create complex techniques. Ambient desorption allows samples to be analyzed rapidly in their native state without preparation. This ability of techniques in providing analyte desorption selectively combined with mass spectrometry has given major alternatives in their areas of application in both qualitative and quantitative analysis in nature. This includes chemistry process, in vivo analysis, pharmaceutical analysis, metabolomics, biological imaging and detection of explosives (Albert 56). New methods of ambient ionization combined with desorption or ionization techniques and hyphenated methods have been created. It increases the number of methods that have been documented to 30. There are many current ambient ionization techniques that if covered it would distract from the objective of the course.
Desorption electrospray ionization
An experiment in desorption electro spray ionization involves ions being from surface of a sample by being bombarded by high velocity, charged droplets in the air. The spray functions to form small droplets in the air that cannot be seen on the surface of the given sample where condensed analyte dissolves. The process that follows is desorption through the transfer of momentum by droplets collide with the liquid that has dissolved into geyser to forcing the analyte that has dissolved into atmospheric air as micron-sized droplets (Gross ,2011). Desorption electric analyzation has a number of applications such as counterfeit ID, degradation studies or direct tablet analysis to chromatography plated and possibility of 2Dmoleculor imaging of biological tissue .
Direct analysis in real time for mass spectrometry is an ambient ionization technique that analyses samples of different sizes, physical states or different shapes. It might require samples or not. The mode operation involves passing of nitrogen or helium through a needle with high voltage and another counter electrode to generate an electric charge. Electrodes are used to also remove unwanted gas that is not the” excited-state gas atoms or molecules”. The excited gas or molecules might be used in the ionization of electrolytes either directly or might interact with the ambient compounds to create what is known as reagent ions that can ionize analyte molecules. The analytes are released into the elements of the excited gas exciting the source of DART of desorption and ionization. Analyses take place in the ambient conditions. Dart is applicable in homeland security, forensic science, pharmaceutical science, explosives, pesticides and illicit drugs and even chemical warfare.
The goal of analytical chemistry is sample analysis without sample preparation analyzing a sample before mass spectrometry in real time spectrometry had to use liquid chromatography for sample analysis in pharmaceuticals. It took time to get the chromatogram and prepare the samples ionization sources have been used with mass spectrum. There are various sources of ionization that were used with a mass spectrometer. Some required samples to be prepared and introducing samples in a high vacuum to be analyzed. This mostly included field desorption or ionization, chemical ionization, electron ionization. The fact that these samples require using a high vacuum for analysis is a great disadvantage because of cases like failure or contamination of the vacuum and source of ionization if the sample is more than is required.
The high vacuum problem was solved by introducing atmospheric ionization sources like the atmospheric pressure ionization (APCI), the electrospray ionization (ESI) 2 and the matrix-assisted laser desorption (MALDI) and Atmospheric pressure photoionization (APPI). The use of atmospheric pressure ionization saw the increase of the compounds to be analyzed through mass spectrometry but samples had to analyzed at higher temperatures, electrical potential, laser, high, ultraviolet irradiation, high velocity gas stream which needed precautions to protect the operator from any danger (Lebedev, 2012). The different techniques of ambient ionization show various characteristics from atmospheric chemical ionization such as sample analysis of objects in an open environment, direct analysis of samples that have not been treated without destroying properties of the sample. Desorption electrospray ionization (DESI) is a form of ambient ionization developed to analyze surface of samples but with an electrical aqueous spray. Samples of DESI are biological samples that cannot be solved by (MALDI) because it also has features of ESI and does not require sample preparation. No matrix is needed. Ambient ionization sources such as DESI and DART are different from other ionization sources because they analyze compounds of low molecular weight on the surface of solids and liquids without prior preparation or separation through chromatography.
Ambient desorption, it must be noted, gives room for an analysis that is more direct of ordinary objects, especially when conducted in the laboratory’s open air. This is also the case when the analysis is conducted in their natural environment. One step that is normally accompanied by the analyte desorption is the ionization step. A keen analysis reveals that these processes are normally multi-step and concerted. Ambient desorption further require very minimal sample preparation, and this probably explains why it preferred. It makes usage of MS very easy, and it also delivers a work flow that is much simplified. Since the DESI was introduced, there has been rapid development in the field of MS which was until recently considered new. There have been numerous permutations, especially when it comes to the various options that can be used for ionization as well as analyte desorption. it must also be noted that there are numerous steps with regards to this which can all be used. They include thermal desorption, dissolution into reverberating droplets, as well as momentum transfer. These desorption steps have in recent times been united with ionizing steps the ionizing steps in question include photo ionization, chemical ionization, ESI, and atmospheric pressure. Recent developments have seen a massive number of ionization and desorption combinations being applied with a view of creating a proliferation of acronyms and techniques.
When the ESI-MS was invented, it came in handy in providing a more conclusive way for separating analytes from matrix that is solution based. This made it even easier to transfer the solutions free ions at atmospheric pressure into the environment that is high-vacuum. For analysis by MS to be successful, such kinds of environments are necessary. The role that MALDI plays with regards to this can also not be underscored. It gives room for the analytes to be analyzed, especially those analytes that have been dispersed in a matrix in a condensed phase. Through these developments, the analysis has been simplified greatly. This has consequently extended the sample types that the MS can interrogate, as well as the ease of use. However, it must be noted that MALDI, ESI, as well as all atmospheric pressure ionization sources that are deemed traditional still need sample preparing steps that are extensive. These include atmospheric pressure photo ionization and atmospheric pressure chemical ionization.
The thorough sample preparing steps are meant to ensure that the dissolution of the sample can be done with a suitable matrix that has been specifically selected. The introductions of DART and DESI were there major breakthroughs in this process. This made it possible for samples to be analyzed directly while they are still in their native condition. It was done by the analytical system elements being bypassed before the ions are transferred into the mass spectrometer. It must be noted that this is a process that can be undertaken without sample preparation and sample manipulation steps. When ambient desorption ionization is carried out in only one operational step, it can successfully bridge the gap that exists in the ambient environment. This is where samples of the condensed phase are present. The analysis takes place in the vacuum system. The development of DESI was instrumental in creating the much needed awareness regarding potential ambient environments. This is what sparked the new MS sub-field. This concept had a novelty that could only be demonstrated by quick introduction of new methods.
It is important that ambient ionization techniques be systematized according to their customary technique. This will ensure that their function in the entire ionization process is preserved, and this will also ensure the resulting mass spectra are governed. This is what enables them to be separated into classes. Examples include those that are related to the ESI closely such as DART, DESI and APCI. Others that are related to this include ASAP, and DAPCI.
It must be noted that when the ionization step is separated from the desorption step, it conceptually aids in rationalizing the contribution that individual techniques have in the entire process. This is especially so when various techniques are combined. Through these methods that a re combined, it becomes easier for spectra to be produced, and a keen analysis reveals that they have a striking resemblance to the ESI that has been observed. The separation of the ionization steps and the desorption is normally not an easy task. This is a process that is often surrounded by numerous uncertainties, because the question of whether the desorption step is involved is never really addressed. This is therefore a demanding process that calls for the gaseous samples to be analyzed by electrospray that is pneumatically assisted.
The role of ESI related techniques can therefore not be underscored in this process. Certain requirements must be ensured before the ionization can occur.
They include deprotonating being ensured in the negative-ion mode, anions and cations being released directly from salts, and adduct formation for negative ions and positive ions. The formation of multi ionization products and solvent clusters ensures the frequent complexity of mass spectra. They include alkali metal Cation adducts and multiply charged ions. In ambient techniques that are ESI related, the desorption of analyte molecules occurs first. They are consequently taken to the mass spectrometer in charged solvent droplets that are evaporating. There are great differences when it comes to the desorption mechanisms. Compared to momentum desorption, they vary greatly. Electrosprays that are pneumatically assisted also come in handy when used to directly sample liquids. This is especially the case in FD-ESI and EESI. In such instances, samples are nebulized pneumatically before being intercepted by ionizing electrosprays. It is from the neutral sample droplets that analytes are extracted, because their solubility is extremely different. The analytes selective extraction is mostly preferred, largely due to the differences that are witnessed when it comes to spatial distribution. When the solid sampling probe is being used, there is the creation of a liquid junction between the solid surface and the probe. It is through solvent flow that the collection of soluble compounds takes place. This is subsequently taken to the electrospray emitter.
The LAESI and ELDI ambient ionization techniques are instrumental when it comes to combining ESI with laser desorption. However, unlike ionization methods that use ambient desorption, an additional matrix is required by MALDESI before any analysis is done. In MALDESI and ELDI, there is need to use a UV laser so as to ensure the analytes are desorbed from the sample surface. An IR laser will be utilized for a similar purpose when it comes to the LAESI. It is the laser type that is used that will determine the degree of selectivity when it comes to the target analyte’s nature. The concept of DESI and SSI were proposed to provide beyond ambient MS analysis with a little or no preparation of samples (Walter, 2009). DESI has founded many applications used widely including direct sample analysis of metabolism substances, drugs, biological samples and explosives. This is because of its easy assembly, fast introduction of samples and robustness combined with high sensitivity, scanning speed and high selectivity of MS. The spray that is applied is a dc electrical potential. The solvent then flows in an inner capillary tube and then the nebulizing gas nebulates it to get small but charged droplets. The spray gas is sprayed through space to increase nebulization through the space of inner and outer capillaries which are actually concentric with each other. The velocity of the spray gas can reach a few hundred meters per second. The solid sample being analyzed is impacted by the droplets which are charged and its analytes are taken up and ionized for MS analysis.
The DESI step is also critical to the entire process. It is the one that determines how aqueous droplets impact sample surfaces, especially those of velocities exceeding 100 m/s. this mechanism hardly depends on the impact of droplet-surface quasi-elastic events. Even the intact droplets survival does not shape the final outcome. There are various assumptions that are made with regards to this. One of them is that the initial droplets make the surfaces wet. Droplets that arrive later also impact this surface by breaking it up before creating various off-spring droplets that have material coming from the solvent layer. This includes the analytes that have been dissolved. It is through momentum transfer that analyte desorption takes place. This comes in the form of droplets that have been charged, which are consequently ionized by ESI mechanisms. In the DeSSI version of DESI, it must be noted that no voltage is applied when it comes to the spray emitter. This implies that solvent droplets will be produced at a charge density that is lower that would normally be the case.
DeSSI and DESI have differences that are similar to those between SSI and ESSI. It is from the ionization step that these mostly occur. Simulations that take place in the DESI process have proven that hydrodynamic forces are instrumental in the desorption of analytes from surfaces. However, there is need for further investigations so at to know the charging effects as far as the desorption process is concerned. There are numerous geometrical factors that shape the signal intensity that will be observed when it comes to the mass spectrometer. Both the distance and the angle relating to the electrospray emitter have to be considered. There is need for the geometrical factors to be re-optimized frequently as this is the only sure way to ensure that a good signal is obtained. This is because the influence that they have on DESI source development is critical, and as such, cannot be ignored. The current DESI source configurations target to minimize the dependence that always existed on the ion signal. Through these configurations, there are numerous benefits that can be realized. They include ease and safety of use, as well as improved ionization efficiency. A positive observation is that performance hardly depends on the spray’s geometrical configurations.
Research shows the essential usefulness of conventional ion sources in reaction monitoring field especially in kinetics reactions study and the intermediates for elucidation of reaction mechanisms. ESI is critical in this field and continues to expand. Monitoring reactions benefit from rapid development of ambient sources of ion or the novel introduction methods of the sample. An example is the set up of DESI. The reaction intermediates are intercepted on the time scale of the millisecond. Real time reaction can monitor sample without sample preparation in a viscous liquid. This is not possible with any other MS method; this concept is because of the interesting system of micro jetting phenomenon. It is clear that obstacles that are encountered using ESI when analyzing unusual compounds and system can be rectified by using a novel source of ion, the objective of the study is to introduce the methods currently available and show how their unique their application can be used. It has been widely demonstrated that that the large monitoring reaction methods that are based on MS can offer flexibility and a convenience in addressing the concerns and problems. Advance in rapid sources of ion together with sample introduction technique will drive the development of this field to greater heights.
Albert, Anastasia. "Chemometric optimization of a low-temperature plasma source design for ambient desorption/ionization mass spectrometry." Spectrochimica Acta Part B: Atomic Spectroscopy ( 2014). Print.
Chang, Cuilan. "Graphene matrix for signal enhancement in ambient plasma assisted laser desorption ionization mass spectrometry." Talanta (2013): 54–59. Print.
Eberlin, Livia S. "Desorption electrospray ionization mass spectrometry for lipid characterization and biological tissue imaging." Biochimica et Biophysica Acta (BBA) - Molecular and Cell Biology of Lipids (2011): 946–960. Print.
García-Reyes, Juan F. "Chapter 8 – The Potential of Ambient Desorption Ionization Methods Combined with High-Resolution Mass Spectrometry for Pesticide Testing in Food." Comprehensive Analytical Chemistry (2012): 339–366. Print.
Huang, Min-Zong. "Ambient molecular imaging of dry fungus surface by electrospray laser desorption ionization mass spectrometry." International Journal of Mass Spectrometry (2012): 172–182. Print.
Venter, Andre. "Ambient Desorption Ionization Mass Spectrometry." Trends In Analytical Chemistry (2011). Print.
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