Interactions with Mineral Surfaces Investigation of DNA adsorption on chlorite and other layered mineralsMuscovite mica is commonly used to immobilize DNA molecules onto a flat surface. This method, however, requires either the use of divalent cations in the buffer solution or the chemical modification of the surface, as both the surface and DNA molecules are negatively charged. DNA molecules have different binding affinities and assume different conformations when adsorbed to different layered minerals. Using atomic force microscopy (AFM) it is has been experimentally confirmed that biotite, talc and brucite have a much higher affinity than muscovite, with 7-, 20- and 25-fold more volume of DNA deposited, respectively.
5 nM of linear DNA from a physiological buffer containing 10 mM MgCl2 deposited onto freshly cleaved (a) muscovite, (b) bitotite, (c) talc and (d) brucite, the white arrows show DNA aggregation. The scale bars are 1 μm. |
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The deposition of DNA onto chlorite presents two distinct areas.
Brucite-like regions have high DNA coverage, and mica-like regions where DNA molecules are
almost absent. In some cases, DNA molecules become stretched across these mica-like regions.
The stretching is driven by the surface potential gradient between the two regions.
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DNA from the above conditions deposited onto freshly cleaved chlorite. DNA is bound to the brucite-like area and stretching events across the mica-like regions are shown. The scale bar is 500 nm. |
The bio-inorganic interface: the interaction of fungal hyphae with mineral surfaces
The weathering of silicate minerals in soil is vital to the formation of clay minerals, e.g.
vermiculite and smectite, and for the release of nutrients, such as K and Mg, into the soil
solution for uptake by plants. It is thought that the plants themselves, along with their
associated symbiotic mycorrhizal fungi, may help to accelerate this process through the release
of organic acid exudates. This may represent an important, and largely neglected, sink for
atmospheric CO2 into the soil. In this project, we will use Atomic Force Microscopy (AFM) to
investigate the degradation and dissolution of mineral surfaces.
Initially, different rock samples have been analysed for topography and mineral composition
with Scanning Electron Microscopy (SEM) and Electron Microprobe (EPMA). Their surfaces have
also been imaged with AFM (the image below shows an example of a chlorite surface). Combining model
experiments exposing mineral surfaces to organic acids, and using in-situ imaging of active
fungal hyphae on mineral surfaces, we will characterise the early-stage dissolution process,
a first step in the weathering of the mineral surface. We will also use other surface-sensitive
techniques, such as x-ray photoelectron spectroscopy (XPS) and secondary ion mass spectrometry
(SIMS), in these experiments. By using the AFM in a force measuring, or force-spectroscopy,
mode we will map the changes in surface properties of both the mineral and the hyphae, for
instance the hydrophobicity/hydrophilicity of the surfaces. The hyphae-mineral interaction is
dynamic and relatively rapid, and we will also use specialist high-speed AFM techniques, which
have been developed in Bristol, to allow us to follow the process in real time.
 Height (left) and phase (right) AFM images of a chlorite surface (width = 5 μm). The phase image reveals the presence of both brucite- and talc-like layers on the surface of the chlorite. |
Hormone adsorption and degradation on mineral surfaces
In recent years, Endocrine Disrupting Chemicals (EDCs) and Pharmaceutically Active Compounds (PhACs)
have become a major aspect in environmental chemistry, marine biochemistry and human health care.
They are able to interfere with hormonic actions, block natural hormones from working or mimic them.
EDCs can derive from several products in agriculture such as fungicides, insecticides, pharmaceuticals
and their metabolites, paint (e.g. anti-fouling on ships) and plasticisers.
Progesterone is the most important and only naturally occurring hormone of the progestagens - its
chemical formula is C21H30O2. The amount of progesterone and other steroid reproductive hormones is
significantly high in the environment and displays a new challenge for scientists. For these
hormonally-active compounds, conventional removal methods, such as chlorination and ozonolysis, have
been used, but little is currently known about either their fate and transport in the environment
nor their degradation products. So far, UV experiments have mainly been carried out in liquid with
other EDCs, hormones and micro-organisms.
The appearance of the deposited hormone was variable depending on the substrate the solution had
been dropped on. On mica, small, closely packed spheres were formed, whereas on graphite, the
hormone crystallised in more or less oriented, elongated crystals. The orientation of these
crystals is presumably linked to the underlying C atom lattice.
 Figure 1. Hormone deposited on mica. |
 Figure 2. Hormone deposited on graphite. |
A 254 nm wavelength UV lamp (1900 μW/cm2) was used to
initiate changes on the mineral surface, followed using AFM. The aim was to degrade the hormone on
the mica/graphite surface, following the changes using AFM. Changes, such as variations in
surface structure, can be seen in AFM, but need to be assessed more precisely with other
methods, as the AFM does not give the required compositional information.
UV experiments included exposure of the sample during scanning as well as intermittent
exposure between successive scans. Here, a sequence of AFM phase images presents the
changes in structure on the HOPG surface as a result of UV exposure.
Figure 3. Changes in structure of hormone on HOPG surface as a result of UV exposure. |
An AFM liquid cell has been used to monitor the adsorption of progesterone onto mica in situ. By imaging the surface in water and subsequently injecting a solution of the hormone, the adsorption process itself can be followed from time zero.
Movie - Hormone adsorption under liquid. |