Lecture 9: Surface Properties & Analysis

surface

• first contact with biological system

• surface of material is different from the bulk

• easily contaminated

surface structure of a material

often mobile, ie. atoms or groups of atoms move according to the environment

parameters to be measured

• wettability (contact angle)

• chemical composition (IR, XPS, SIMS)

• topography and roughness (SEM, AFM)

• electrical charge

• surface mobility

• crystallinity

• heterogeneity

wettability: hydrophobic surfaces

PE, PTFE, low surface energy , water beads at the surface

wettability: hydrophilic surfaces

polar/hydrophilic groups: -COO, -NH4+, -OH, -COOH, -C=O, -NH2

hydrophilic surfaces have high surface energy, water wets the surface

wettability: surface structure

surface structure can be mobile, ie. hydrophilic domains or groups may rearrange to face the surface in an aqueous environment

contact angle

describes the shape of a liquid drop resting on a solid surface; measure of the liquid wetting of a surface, an estimate of surface energy, which in turn is a characteristic of chemical bonding

higher contact angle…

lower surface energy/surface tension

high water contact angle (WCA)…

surface is hydrophobic

low WCA surface is…

hydrophilic

contact angle measurement concerns

• operator dependent

• surface roughness

• surface heterogeneity

• liquid contamination

• liquid changes the surface (reorientation, absorption, swelling, dissolving)

highest water contact value

PTFE (112)

lowest water contact value

PET-PEG (19)

surface with a high water contact angle is best described as

hydrophobic with low surface energy

fourier transform infrared spectrum (FTIR)

IR spectroscopy measures the vibrations of chemical bond; each bond vibration absorb incoming IR energy at particular frequency

how FTIR works

infrared spectometer sheds IR beam on the sample and measures the amount of radiation at various wavelengths that is transmitted or reflected by the sample

FT turn raw data into

spectrum

chart of characteristic IR absorption bands

shifts in the frequency of absorption bands and changes in relative band intensities indicate changes in the chemical structure or changes in the environment around the sample

absorption band

assigned to functional groups

wavenumber (cm^-1)

more frequently used in IR spectrum

wave number = 1/λ

wave length = λ

ATR - FTIR

provides more surface specific information (1-5 um)

ATR - FTIR advantages

• inexpensive

• quick

• no sample preparation

• can look at kinetics (such as protein adsorption)

ATR - FTIR disavantages

• not highly surface sensitive

• have to make really good contact with the IRE, need flat surface

X-ray photoelectron spectroscopy (XPS)

based on the photoelectric effect, interaction of the x-rays with the atoms cause the emission of inner shell electron; the kinetic energy of electron is measured; the binding energy can be calculated and used as the characteristic parameter of specific element

• extremely surface selective and sensitive

• ESCA

ESCA is very sensitive

• although X-ray beam can penetrate deeply in the specimen, electrons emitted deep in the specimen (D, E, F, G) will lose their energy in inelastic collisions and never emerge from the surface

• only electrons emitted near the surface (A, B), lose no energy and will contribute to the ESCA signal

• electrons that lose some energy but still emerge from the surface (C) contribute to the background signal

ESCA advantages

• surface sensitivity

• high info content

• depth profile

• spatial resolution

• low damage potential

• little sample preparation

ESCA disadvantages

• expensive facility and training

• need vacuum compatibility

• not good for complex surface

secondary ion mass spectrometry (SIMS)

• based on the generation of (secondary) ions by bombardment of a solid surface by incident beam of accelerated ions (primary ions)

• mass/charge of the secondary ions are measured using a time of flight mass analyzer

dynamic SIMS

• used for obtaining compositional infor as a function of depth below the surface

• high ion doses

• surface erodes fast

• only atomic fragments can be detected

• more artifacts as the beam erodes faster

static SIMS

• used for sub-monolayer elemental analysis

• low ion dose, it’s adjusted so that less than one monolayer of surface atoms is sputtered

• large fragments can be ejected and analyzed

SIMS advantage

• most surface sensitive (15A or smaller)

• useful with isotopes

• can achieve high spatial resolution (40 nm or less) by focusing primary ion beam magnetically or electrostatically

• imaging SIMS

SIMS disadvantages

• expensive

• complex interpretation

scanning electron microscopy

focusing and rastering a high energy electron beam on a specimen; detecting the emitted low energy secondary electrons; reconstructive the intensity of the secondary electrons on a phosphor screen

scanning electron microscopy for conductive samples

energy dispersive x-ray (EDX) analysis can be used for bulk elemental analysis (sampling from a few micron deep)

scanning electron microscopy for non-conductive samples

coated with thin electrically grounded layers (metal or carbon) to minimize the negative charge accumulation, surface chem info will be contaminated

scanning electron microscopy measurements

good for qualitative surface roughness and texture

low-voltage or environmental SEM (ESEM)

permits the use of uncoated specimen; ESEM allows wet sample measurement

atomic force microscope

measures topography with a force probe

atomic force microscope laser beam reflection

offers a convenient and sensitive method of measuring cantilever deflection

AFM cantilevers

have ranges of spring constants

micro machining techniques

produce inexpensive, reasonably sharp lines

piezotube

positions the sample with high resolution

atomic force microscopy

can be used with both conductive and non-conductive surfaces, can be used under water, in air and vacuum

atomic level resolution but usually can not be obtained for organic and biological samples

atomic force microscopy limitations

• tip shape limitation

• damage and pushing around of surface structure caused by the tip

atomic force microscopy modes

contact vs tapping mode decreases sample damage