«By Yusuf Nur A thesis submitted to The University of Birmingham for the Degree of DOCTOR OF PHILOSOPHY School of Geography, Earth and Environmental ...»
4.2 Synthesis of gold nanoparticles This section introduces different synthesis methods used in this project to produce high quality gold nanoparticles with different sizes and different surface chemistry. To modify the surface chemistry of the nanoparticles two different coating agents were used as will be explained in the following sections.
The following chemicals were of analytical grade and have been purchased from Sigma Aldrich: tetrachloroauric acid (HAuCl4.3H2O), trisodium citrate dehydrate, NaOH, HNO3, HCl, NaCl, Polyvinyl pyrrolidone (molecular weight =10,000; hereafter referred to PVP) and were used as received from the supplier without further purification. The water used for all reactions and preparation procedures were milli-Q quality R 18 MΩ cm, and was obtained by filtering through Millipore cartridges. All glassware used in these synthesis processes were thoroughly cleaned three times in aqua regia (3 parts HCl: 1 part HNO3), rinsed with ultra high purity water (resistivity 18.2 MΩ·cm) and air dried.
4.2.2 Synthesis of AuNPs capped with Citrate AuNPs were prepared by citrate reduction of HAuCl4.3H2O according to the following procedure which is based on the Turkevich method(Turkevich, 1951) as modified by Frens (Frens, 1973) and others (Kumar et al., 2006).The required volume of an aqueous solution of HAuCl4.3H2O was heated to the boiling point while stirring vigorously and then trisodium citrate was added quickly. The actual concentration of the solutions are summarised in Table 4-1. The ratio of the two chemicals was varied in order to produce the size of the desired nanoparticles (Table 4-1). An immediate colour change from pale yellow to deep red occurred within 10 minutes. The solutions were kept at the boiling point for 15 minutes to assure the completion of the reaction and finally allowed to cool to room temperature.
4.2.3 Synthesis of AuNPs capped with PVP AuNPs stabilised by PVP were prepared using two different synthesis methods. The
of gold and PVP solutions up to 70 o C and it will be called hot method hereafter while the other synthesis method was carried out at room temperature and it will be referred to as cold method.
220.127.116.11 Cold process.
A detailed method of the process was given elsewhere (Zhou et al., 2009). Briefly, an aqueous solution of tetrachloroauric acid was added to a solution of polyvinylpyrrolidone (PVP) under vigorous stirring in room temperature. Then aliquots of NaOH, whose concentration was accurately known, was added to initiate the reduction of the gold ions which then was capped and stabilised by PVP (see Table 4-1). To achieve different sizes of AuNPs the ratio of gold precursor to capping agent were altered.
18.104.22.168 Hot process.
In preparing the nanoparticles of gold, the general procedure followed is summarised below.
A known quantity of tetrachloroauric acid was weighed and dissolved in water. Then various amounts of PVP solution were added, as summarised in Table 4-1, to achieve different sizes of AuNPs. The final solution of HAuCl4.3H2O and PVP in water was heated up to 70 oC for 3 hours. After cooling to room temperature, the solutions was washed with acetone and water with ratio (3 acetone: 1 water) and centrifuged 10 min at 4000 rpm to get rid of the access PVP. The so produced AuNPs were then redispersed in water and filtered with 100 nm filter paper.
This section introduces pre analysis cleaning and preparative size separation techniques used to prepare nanoparticles and to clean all glassware used during the synthesis. Any excess capping agents and unreacted precursor ions need to be separated from NPs before they are analysed and characterised since they either may affect the quality of the analysis or may complicate the interpretation of the toxicity data of the nanoparticles. Similarly, bacterial cells need to be separated from the extracellular polymeric substances (EPS) prior to their fixation and sectioning for further imaging with transmission electron microscope (TEM) (see section 3.1.2).
4.3.1 Glassware Cleaning All plastic and glassware used were treated with 10% nitric acid solution and thoroughly washed three times with utra high purity water (18 cm-1) to prevent contamination between samples and between successive synthesis processes. The plastic and glassware were air dried prior to their usage. Gold nanoparticles adhere to reaction vessel walls strongly, so an aqua regia (1:3 concentrated nitric to hydrochloric acid) solution was used for their removal.
4.3.2 Ultrafiltration Ultrafiltration is a separation method that uses pressure to squeeze liquid and dissolved chemicals through a porous membrane while molecules and particles which are bigger in size than the membrane pores are retained. Since the separation is based on the pore size, it can be used to achieve a size selected separation for cleaning and further analysis purposes (Timmer
ultrafiltration membrane polysulphone and cellulose acetate are the most common materials (Dhawan, 2012). Ultrafiltaration process can be used to clean up and remove excess reactants and to quantify dissolution.
The ultrafiltration system used in this project is Millipore stirred ultrafiltration cells model 8400 which can filter a liquid volume of up to 400 ml. A pressurized nitrogen gas of 25psi is used to drive the liquid through a cellulose membrane with molecular weight cut off of 1kD.
Samples were washed to remove gold ions by ultrafiltering 50 ml of AuNPs suspension. The volume was reduced to 25 ml and was replenished to 50 ml. This process was repeated three times with 1 hour interval between the subsequent filtration to allow the NPs to reach equilibrium. For each samples replicates of unfiltered sample for total concentration and filtrate for dissolved ions were collected and acidified with concentrated nitric acid for further analysis of gold concentration by inductively coupled plasma mass spectometry ICP-MS (see section 3.1.8). The average mean values of each replicate were reported and the gold NP concentrations were calculated by difference. The analysis was performed using an Agilent 7500 ICP-MS instrument housed in an air conditioned room.
4.4 Characterisation of the synthesised gold Nanoparticles (AuNPs) A number of analytical and imaging techniques have been used to fully characterise the synthesised AuNPs. The following subsections will introduce how these techniques were applied and the related sample preparation steps.
Centrifugation technique (an Eppendorf 5804 R) was used with maxmimum speed 5000 rpm to clean samples by removing access coating agents and to achieve size based separation of the nanoparticles. It is also used to separate bacterial cells from the growth media for further analysis such as fixation and sectioning.
Ultracentrifugation technique (Beckman coulter type SW40 with average rotation axis (rmax) of 112.7 mm and K factor of 137) was used as preparation steps prior to the imaging of the particles with TEM (see section 3.1.2 below) and AFM (see section 3.1.3). Either mica sheet or TEM grid is immersed inside ultracentrifuge tube containing highly diluted samples followed by one hour spinning process with 30,000 rpm. This extremely high rotational speed drives all particles on the surface of the mica/grid at the bottom of the ultracentrifuge tube.
4.4.2 Imaging NPs with Transmission Electron Microscopy (TEM) In this project JEOL1200 TEM which has a resolution of up to 0.45 nm was used to visualize both the gold nanoparticles and bacteria samples. It produces bright dark field images and the acceleration voltage of this instrument can be varied from 60 V to 120 V. for most of the samples 80 V were operated. Images were collected digitally using a Gatan Dual Vision 300W digital camera. Nanoparticles were immobilised on a 300 mesh carbon-coated copper grid by immersing the grid in diluted solutions of nanoparticles and ultracentrifuging at 150,000 g for 1 hour. Subsequently, grids were washed with water of ultra high purity and left to dry at room temperature overnight prior to TEM analysis. For the simple systems (i.e.
NP suspension at low ionic strength) measured here, this preparation method was suitable,
(Baalousha et al., 2012b, Baalousha et al., 2012a). Sections on the grid were randomly chosen and all NPs located within those sections were counted. Mean particle size, shape factor and associated standard deviations were calculated from the observation of at least 100 particles using Gatan digital micrograph software. The shape factor of particles was calculated using Equation 4-1 below.
Where P stands for perimeter and A for the area of the particles.
Additional size measurements and energy dispersive x-ray (EDX) analysis were carried out on a JEOL 2100 analytical TEM, which has a LaB6 electron gun and can be operated between 80 and 200kV. This instrument has a resolution of 0.19nm, an electron probe size down to 0.5 nm and a maximum specimen tilt of ±10 degrees along both axes. The instrument is equipped with an Oxford Instruments LZ5 windowless energy dispersive X-ray spectrometer (EDS) controlled by INCA software for the analysis of the EDX data.
4.4.3 Topography of the NPs measured with atomic force microscopy (AFM) In this project, XE100 (AFM) from Park System was used to image the gold nanomaterials (see section 3.1.3 for theoretical description of the AFM) adsorbed on freshly cleaved mica sheet of 1 cm2 surface. Prior to the measurement, the mica sheet was immersed in the AuNP solution in a centrifuge tube followed by ultracentrifugation process with 30,000 rounds per minute (rpm) for 1 hr. After that, it was gently rinsed with milli-q water and air dried for further analysis. The topographical sizes of more than 100 randomly selected particles were
the instrument to study the topography and the shape of the particles and to estimate their size distribution.
4.4.4 Hydrodynamic and zeta potential measurement with dynamic light scattering (DLS) The size and the zeta potential of gold nanoparticles synthesised during this project were continuously measured using dynamic light scattering from Malvern (see section 3.1.4) (HeNe laser Malvern High performance Particle Sizer Model NanoZS90 HPPS 5001). This instrument measures the hydrodynamic diameter of sphere which diffuses the same speed as the nanoparticles measured (Malvern, 2007). Since the diffusion rate depends not only on the core size of the particles but also any surface structures including the capping agents, the size measured with DLS will be larger than the size of the same particle measured with electron microscopy (Malvern, 2012). The measurement of the size was performed by filling a plastic disposable cuvet with the sample. It takes about 2 minutes to optimize the measurement conditions and to stabilize the temperature of the sample around 25 0C (Malvern, 2004). Five replicates were measured and their average values were reported.
Apart from the size of individual NPs and zetapotential of the particles, this instrument was used to monitor the aggregation of AuNPs in different relevant media including the growth media of the bacteria and aqueous media of different ionic strength. In the case of the nanoparticles synthesized in this project where water media is used and particles sizes in lower nanometer scale Henery’s function (see section 3.1.5) is applied to calculate zeta potential from electrophoretic mobliliy measured with DLS. The nanosizer software controls the instrument and allows for analyzing the data to either drive the size or the zeta potential information of the particles.
In this project, the localised surface Plasmon resonance ( see section 3.1.6 for the theory) of the freshly synthesised gold nanoparticles were recorded to study the presence of gold which has its characteristic absorption peak at around 520 nm(Daniel and Astruc, 2003, LizMarzÃ¡n, 2005). The stability of NPs over a long period of time in both capping agents solution (6 months) and two weeks in Minimal Davis Media -which is the growth media of bacteria- were studied by recording their LSPR spectrum. The LSPR of gold nanoparticles of different sizes has also been recorded and compared among them to study any red shift of the maximum absorbance wave length of the gold particles in the sample due to the different sizes of the particles.
The instrument used for the measurement, collection and data analysis is 6800 Uv-vis spectrometer from Jenway (Figure 4-1 below). The main parts of this instrument are light source, sample holder, monochromator to select different wavelength of light, filter, detector and output (computer). It is a double beam spectrophotometer with highly stable optics of
1.5 nm spectral bandwidth. Jenway Flight Deck software was provided with it to collect, record, and display and analyse data. Solution spectra were obtained by measuring the absorption of dilute particle suspension at a wavelength in the range 200-700 nm in a quartz cell of 1 cm path length.
FFF was one of the size measurement techniques used in this project to measure the hydrodynamic diameter of the gold nanoparticles synthesised in this project. Separation and sizing of gold nanoparticles were performed in an asymmetrical flow field-flow fractionation (from Postnova Analytics, Germany). 2 mM NaNO3 solution was freshly prepared and used as carrier solution in this analysis. UV spectrometer was used for the detection of both the polystyrene standards (at 250 nm) and the eluated AuNPs (at 520 nm). Samples were diluted several times to verify the overloading effects of the samples. At least three independent replicates were analysed per sample and the data averaged.