We introduce an approach to synthesize rare earth oxide nanoparticles using high temperature without aggregation of the nanoparticles. The dispersity of the nanoparticles is controlled at the nanoscale by using small organosilane molds as reaction containers. Zeptoliter reaction vessels prepared from organosilane self-assembled monolayers (SAMs) were used for the surface-directed synthesis of rare earth oxide (REO) nanoparticles. Nanopores of octadecyltrichlorosilane were prepared on Si(111) using particle lithography with immersion steps. The nanopores were filled with a precursor solution of erbium and yttrium salts to confine the crystallization step to occur within individual zeptoliter-sized organosilane reaction vessels. Areas between the nanopores were separated by a matrix film of octadecyltrichlorosilane. With heating, the organosilane template was removed by calcination to generate a surface array of erbium-doped yttria nanoparticles. Nanoparticles synthesized by the surface-directed approach retain the periodic arrangement of the nanopores formed from mesoparticle masks. While bulk rare earth oxides can be readily prepared by solid state methods at high temperature (>900 °C), approaches for preparing REO nanoparticles are limited. Conventional wet chemistry methods are limited to low temperatures according to the boiling points of the solvents used for synthesis. To achieve crystallinity of REO nanoparticles requires steps for high-temperature processing of samples, which can cause self-aggregation and dispersity in sample diameters. The facile steps for particle lithography address the problems of aggregation and the requirement for high-temperature synthesis.

One contribution of 10 to a Theme Issue 'Molecular-, nano-and micro-devices for real-time in vivo sensing'. We introduce an approach based on particle lithography to prepare spatially selective surface platforms of organosilanes that are suitable for nanoscale studies of protein binding. Particle lithography was applied for patterning fibrinogen, a plasma protein that has a major role in the clotting cascade for blood coagulation and wound healing. Surface nanopatterns of mercaptosilanes were designed as sites for the attachment of fibrinogen within a protein-resistant matrix of 2-[methoxy( polyethyleneoxy)propyl] trichlorosilane (PEG-silane). Preparing site-selective surfaces was problematic in our studies, because of the self-reactive properties of PEG-organosilanes. Certain organosilanes presenting hydroxyl head groups will cross react to form mixed surface multi-layers. We developed a clever strategy with particle lithography using masks of silica mesospheres to protect small, discrete regions of the surface from cross reactions. Images acquired with atomic force microscopy (AFM) disclose that fibrinogen attached primarily to the surface areas presenting thiol head groups, which were surrounded by PEG-silane. The activity for binding anti-fibrinogen was further evaluated using ex situ AFM studies, confirming that after immobilization the fibrinogen nanopatterns retained capacity for binding immunoglobulin G. Studies with AFM provide advantages of achieving nanoscale resolution for detecting surface changes during steps of biochemical surface reactions, without requiring chemical modification of proteins or fluorescent labels.

SummaryThe solution self-assembly of multidentate organothiols onto Au(111) was studied in situ using scanning probe nanolithography and time-lapse atomic force microscopy (AFM). Self-assembled monolayers (SAMs) prepared from dilute solutions of multidentate thiols were found to assemble slowly, requiring more than six hours to generate films. A clean gold substrate was first imaged in ethanolic media using liquid AFM. Next, a 0.01 mM solution of multidentate thiol was injected into the liquid cell. As time progressed, molecular-level details of the surface changes at different time intervals were captured by successive AFM images. Scanning probe based nanofabrication was accomplished using protocols of nanografting and nanoshaving with n-alkanethiols and a tridentate molecule, 1,1,1-tris(mercaptomethyl)heptadecane (TMMH). Nanografted patterns of TMMH could be inscribed within n-alkanethiol SAMs; however, the molecular packing of the nanopatterns was less homogeneous compared to nanopatterns produced with monothiolates. The multidentate molecules have a more complex assembly pathway than monothiol counterparts, mediated by sequential steps of forming S–Au bonds to the substrate.

Force modulation microscopy (FMM) is a mode of scanning probe microscopy that can be used to visualize changes of tip−sample interactions for hard and soft areas of samples such as polymers and organic thin films. In designed experiments, polystyrene-encapsulated cobalt nanoparticles were imaged with FMM using a home-built sample stage for sample actuation. Regions of the outer polymer coating and the inner cobalt nanoparticle were resolved with high resolution. Using FMM, differences in the elastic and viscoelastic properties of the nanoparticles were visualized with nanoscale resolution by monitoring the return amplitude and phase signals as the AFM tip is scanned over areas of a sample. Regions of the sample with greater elasticity and viscoelasticity generate a weaker signal relative to harder areas because more of the energy associated with the cantilever oscillation is dissipated by the material. Areas with greater elasticity will tend to absorb more of the energy of the cantilever causing the amplitude of the oscillation to be dampened. Conversely, harder areas, having a lower elasticity, will cause the tip to oscillate closer to the input driving amplitude of the piezoceramic. The polymer-encapsulated nanoparticles were patterned using two-particle lithography to prevent aggregation of the nanoparticles.

A hybrid imaging mode was developed for characterizing samples of magnetic FeNi 3 nanoparticles, which combines contact-mode atomic force microscopy (AFM) with magnetic modulation of samples. For conventional magnetic imaging modes of AFM, magnetically coated tips are used directly as a sensor to measure the relatively long-range forces of magnetic samples in a noncontact configuration. For the magnetic sample modulation (MSM) configuration, however, the changes in sample dynamics form the basis for measurements of material properties using contact-mode AFM. Nanoparticles are driven to vibrate in response to an externally applied electromagnetic field, and a nonmagnetic tip is used as a motion sensor for directly mapping the vibration with contact-mode. Intermetallic nanoparticles of FeNi 3 were used as a model nanomaterial, synthesized by either conventional oven heating or microwave preparation. By slowly scanning an AFM probe across vibrating nanoparticles, changes in the frequency and amplitude of the sample motion can be sensitively tracked by the deflection of an AFM probe. Thus, the nonmagnetic AFM tip provides a force and motion sensor for mapping the vibrational response of magnetic nanomaterials at the level of individual nanoparticles. Dynamic protocols were developed for systematic studies with changes in the magnetic field strength and field frequency.