VERTICAL PT-C REPLICATION FOR TEM, A REVOLUTION IN IMAGING NONPERIODIC MACROMOLECULES, BIOLOGICAL GELS AND LOW-DENSITY POLYMER NETWORKS

Vertical replication for TEM is ideal for studying non-periodic specim ens from 0.7 to 3 nm, a resolution mid-range difficult to attain by an y other technique. This paper discusses the importance of vertical rep lication, its methods and hardware for high-resolution TEM. Evidence f rom diverse published research will demonstrate vertical replication's versatility in imaging the molecular level normally unattainable in f reeze-dried polymers, polyethylene tribological wear on surfaces, low- density polymer networks or biological gels. Vertical platinum-carbon (Pt-C) replication minimizes the horizontal movement of Pt-C on a surf ace. Surface objects are symmetrically enlarged by a vertically deposi ted Pt-C film. To estimate real size in replicas, 16-25 particles or f ilaments need to be measured in calibrated transmission electron micro scopy (TEM) images and reduced by a value less than the Pt-C film thic kness measured with a quartz monitor. Continuous, vertically deposited Pt-C films are formed on mica at a deposition thickness of around 1.0 nm and on silver at a thickness of 0.4-0.5 nm. The distance between h elical turns in poly(1-tetradecene sulfone) of 0.7 nm is the highest r esolution achieved with vertical replication. Two polysulfones freeze- dried and vertically replicated on mica contained structures are predi cted by indirect physical chemical methods to be present in solution. Polymer chains are fully Pt-C coated, with no uncoated gaps along chai ns. Some side-chains on the extended non-helical poly(1-tetradecene su lfone) are also detected. To estimate the real chain width, polymer ch ains measured in images are reduced by the Pt-C film thickness minus 0 .5 nm. The polymer chain widths estimated from molecular models are in the same range of widths as those measured using the image size corre ction method. Also, it is possible to distinguish random coil proteins (chain width of around 0.5 nm) from an alpha-helix (chain diameter of about 1 nm) in vertically replicated samples on silver substrates. In the future, subnanometer resolutions below 0.7 nm should be possible. The resolution of vertical replication depends on the thickness of a continuous, amorphous Pt-C film. That thin, continuous 0.4-0.5 nm Pt-C films on silver substrates can be made suggests that a point to-point resolution limit of around 0.28 nm in TEM may ultimately be approacha ble with replication. (C) 1998 Elsevier Science Ltd. All rights reserv ed.