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Cantilever devices have found applications in numerous scientific fields and instruments, including the atomic force microscope (AFM), and as sensors to detect a wide range of chemical and biological species. The mechanical properties, in particular, the spring constant of these devices is crucial when quantifying adhesive forces, material properties of surfaces, and in determining deposited mass for sensing applications. A key component in the spring constant of a cantilever is the plan-view shape. In recent years, the trapezoidal plan-view shape has become available since it offers certain advantages to fast-scanning AFM and can improve sensor performance in fluid environments. Euler beam equations relating cantilever stiffness to the cantilever dimensions and Young&rsquo, s modulus have been proven useful and are used extensively to model cantilever mechanical behaviour and calibrate the spring constant. In this work, we derive a simple correction factor to the Euler beam equation for a beam-shaped cantilever that is applicable to any cantilever with a trapezoidal plan-view shape. This correction factor is based upon previous analytical work and simplifies the application of the previous researchers formula. A correction factor to the spring constant of an AFM cantilever is also required to calculate the torque produced by the tip when it contacts the sample surface, which is also dependent on the plan-view shape. In this work, we also derive a simple expression for the torque for triangular plan-view shaped cantilevers and show that for the current generation of trapezoidal plan-view shaped AFM cantilevers, this will be a good approximation. We shall apply both these correction factors to determine Young&rsquo, s modulus for a range of trapezoidal-shaped AFM cantilevers, which are specially designed for fast-scanning. These types of AFM probes are much smaller in size when compared to standard AFM probes. In the process of analysing the mechanical properties of these cantilevers, important insights are also gained into their spring constant calibration and dimensional factors that contribute to the variability in their spring constant.

The authors describe the synthesis of water-soluble and fluorescent graphene oxide quantum dots via acid exfoliation of graphite nanoparticles. The resultant graphene oxide quantum dots (GoQDs) were then modified with folic acid. Folic acid receptors are overexpressed in cancer cells and hence can bind to functionalized graphene oxide quantum dots. On excitation at 305 nm, the GoQDs display green fluorescence with a peak wavelength at ~520 nm. The modified GoQDs are non-toxic to macrophage cells even after prolonged exposure and high concentrations. Fluorescence lifetime imaging and multiphoton microscopy was used (in combination) to image HeCaT cells exposed to GoQDs, resulting in a superior method for bioimaging. Graphical abstract Schematic representation of graphene oxide quantum dots, folic acid modified graphene oxide quantum dots (red), and the use of fluorescence lifetime to discriminate against green auto-fluorescence of HeCaT cells.

Heterogeneity is an inherent property of a wealth of real-world nanomaterials and yet rarely in the reporting of new properties is its effect sufficiently addressed. Graphene quantum dots (GQDs) – fluorescent, nanoscale fragments of graphene - are an extreme example of a heterogeneous nanomaterial. Here, top-down approaches – by far the most predominant – produce batches of particles with a distribution of sizes, shapes, extent of oxidation, chemical impurities and more. This makes characterization of these materials using bulk techniques particularly complex and comparisons of properties across different synthetic methods uninformative. In particular, it hinders the understanding of the structural origin of their fluorescence properties. We present a simple synthetic method, which produces graphene quantum dots with very low oxygen content that can be suspended in organic solvents, suggesting a very pristine material. We use this material to illustrate the limitations of interpreting complex data sets generated by heterogeneous materials and we highlight how misleading this “pristine” interpretation is by comparison with graphene oxide quantum dots synthesized using an established protocol. In addition, we report on the solvatochromic properties of these particles, discuss common characterization techniques and their limitations in attributing properties to heterogeneous materials. Refereed/Peer-reviewed

We report a novel, versatile method for generating number density gradients of individual gold (Au) and silver (Ag) nanoparticles by a two-step method. First, a chemical gradient of amine surface functional groups is deposited by plasma copolymerization. Second, a density gradient of nanoparticles is formed by the immersion of the chemical gradient in solutions of nanoparticles. Chemical characterization by X-ray photoelectron spectroscopy and morphological analysis by atomic force microscopy shows that nanoparticle density closely follows the change in nitrogen surface concentration across the gradients. We also demonstrate that it is possible to control the slope of the gradients by using nanoparticle solutions of different concentrations. Important for technological and research applications, this method can be used with nanoparticles of various sizes and different materials. In addition, the use of plasma deposition allows such gradients to be generated on any type of substrate. Refereed/Peer-reviewed

Advanced approaches to direct the differentiation of embryonic stem cells are highly sought after. The surface-bound chemical gradient format is a powerful screening approach that can be deployed to study changes in stem cell behavior as a function of subtle changes in surface chemistry. Here, we investigate the spontaneous differentiation of cells derived from differentiating mouse embryoid body (mEB) cells into endoderm, mesoderm, and ectoderm following culture on surface-bound gradients of chemical functional groups in the absence of differentiation-biasing bioactive factors. Gradients were created using a diffusion-controlled plasma polymerization technique. The generated coating ranged from hydrophobic 1,7-octadiene (OD) plasma polymer at one end of the gradient to a more hydrophilic allylamine (AA) plasma polymer on the opposite end. The gradient surface was divided into seven equal regions of progressively increasing AA plasma polymer content and mEB cell response within these regions was compared. Cells adhered preferentially to the central regions of the gradient; however, cell proliferation increased toward AA-plasma-polymer-rich end of the gradient. Variation in the expression of germ layer markers was noted across the gradient surface. High AA:OD plasma polymer ratios triggered cell differentiation toward both mesoderm and ectoderm. Expression of tissue-specific markers, in particular, KRT18, AFP, and TNNT2, was strikingly responsive to subtle changes in surface chemistry, exhibiting vastly different expression levels between adjacent regions. Our results suggest that the surface-bound gradient platform is well suited to screening surface chemistries for use in the field of stem cell technologies and regenerative medicine.

Recently, surface gradients have attracted significant interest for various research and technological applications. In this paper, we report a facile and versatile method for generating surface gradients of immobilized nanoparticles, nanotopography and ligands that is independent from the substrate material. The method consists of first depositing a functional polymer layer on a substrate and subsequent time controlled immersion of this functionalized substrate in solution gold nanoparticles (AuNPs), silver nanoparticles (AgNPs) or poly (styrenesulfonate) (PSS). Chemical characterization by X-ray Photoelectron Spectroscopy (XPS) and morphological analysis by Atomic Force Microscopy (AFM) show that the density of nanoparticles and the concentration of PSS across the surface increases in a gradient manner. As expected, time of immersion determines the concentration of surface bound species. We also demonstrate the generation of surface gradients of pure nanotopography. This is achieved by depositing a 5 nm thick plasma polymer layer on top of the number density gradient of nanoparticles to achieve a homogeneous surface chemistry. The surface independent approach for generation of surface gradients presented in this paper may open opportunities for a wider use of surface gradient in research and in various technologies. Refereed/Peer-reviewed

Smart materials that can sense and respond to changes in the environment are of interest in numerous and diverse applications. In this paper, we report gradient surfaces where wettability and surface potential respond to changes in the pH. The gradients are produced by controlling the concentration of amine and carboxyl acid groups across the surface. The response of surface wettability to pH changes was studied by water contact angle measurements. The potential across the surface was determined by atomic force microscopy-based surface force measurements. These studies showed that at low pH the surface potential changes from ‘‘no charge’’ at the acid end to a positive charge at the amine end. At high pH the surface potential changed from negative at the acid end to ‘‘no charge’’ at the amine side. At an intermediate pH the charge across the surface changes from negative at the acid end to positive at the amine end. Potential applications include separation or guidance of charged entities such as particles, proteins or bacteria. Refereed/Peer-reviewed

Nanoparticle metrology with atomic force microscope (AFM) aims to determine an average particle size from measurements of individual nanoparticles derived by image analysis. This constrains the statistical relevance of the measurement due to the limited number of particles which can be practically imaged and analyzed. Consequently,the number density of particles on samples prepared for particle measurement is an important parameter of sample preparation. A number density that is too low makes it difficult to obtain sufficient measurement statistics, whereas a number density that is too high can result in particle agglomeration on the substrate and limit the area of uncovered substrate that is required to obtain a reliable reference for measuring the particle height. We present imaging and measurement results of a particle number density gradient of 16 nm gold nanoparticles deposited using a gradual immersion process. Results demonstrate how samples with particle number density gradients can facilitate identification of an area on a sample with optimal particle number density for AFM particle metrology and thereby improve measurement efficiency and reliability Refereed/Peer-reviewed

In this paper, we report the initial response of 3T3 fibroblast and MG63 osteoblast cells to engineered nanotopography gradients of three nanoparticle diameters (16 nm, 38 nm and 68 nm). These nanoengineered surfaces were designed to provide a range of nanoparticle densities and comparable surface area across the gradients of different nanoparticle sizes. Importantly, we provided a uniform surface chemistry in order to be able to examine the effect of pure surface nanotopography. We found that nanotopography features of 16 nm encourage the adhesion of both cell types and that there is a critical nanoparticle density between 50 and 140 particles per μm2 where cells adhered in the greatest numbers. When nanotopography features increased to 38 nm the 3T3 cells adhered and spread well, however, the MG63 cells adhered and spread poorly. Both cell types adhered in lower numbers when the nanotopography feature size increased to 68 nm. This work demonstrates that there is a specific nanotopography scale that encourages cell adhesion and spreading, however, the preferential lateral spacing and height of the nanotopography is different for different cell types. Refereed/Peer-reviewed