Romana Schirhagl

Nitrogen-Vacancy Centers in Diamond: Nanoscale Sensors for Physics and Biology

Crystal defects in diamond have emerged as unique objects for a variety of applications, both because they are very stable and because they have interesting optical properties. Embedded in nanocrystals, they can serve, for example, as robust single-photon sources or as fluorescent biomarkers of unlimited photostability and low cytotoxicity. The most fascinating aspect, however, is the ability of some crystal defects, most prominently the nitrogen-vacancy (NV) center, to locally detect and measure a number of physical quantities, such as magnetic and electric fields. This metrology capacity is based on the quantum mechanical interactions of the defects spin state. In this review, we introduce the new and rapidly evolving field of nanoscale sensing based on single NV centers in diamond. We give a concise overview of the basic properties of diamond, from synthesis to electronic and magnetic properties of embedded NV centers. We describe in detail how single NV centers can be harnessed for nanoscale sensing, including the physical quantities that may be detected, expected sensitivities, and the most common measurement protocols. We conclude by highlighting a number of the diverse and exciting applications that may be enabled by these novel sensors, ranging from measurements of ion concentrations and membrane potentials to nanoscale thermometry and single-spin nuclear magnetic resonance.

Applications of Molecularly Imprinted Polymer Nanoparticles and Their Advances toward Industrial Use: A Review

Artificial receptors exhibit several advantages compared to their biological counterparts such as stability, robustness and handling. Additionally, nanostructured materials exhibit excellent properties such as high surfaceto-volume ratio, low cost and easy preparation and handling. Consequently, the approach of molecularly imprinting has been extended from bulk materials to nanomaterials. We are reviewing the numerous applications of molecularly imprinted polymer (MIP) nanoparticles (NPs) and briefly discuss their synthesis. Furthermore, we discuss how and for which applications industry starts to use MIP NPs and what the current limitations are. MIP NPs can be applied as antibody replicae economizing template molecules and facilitating storage. Moreover, nanocomposite materials possess additional functionalities such as magnetic and semi-conductive cores. Nanosized MIPs are widespread applied for analytical separation via solid phase extraction (SPE), but also improves sensitivity in capillary electrophoresis (CE). Furthermore, such nanomaterials are applied in the field of sensors, e.g. in combination with transducers such as surface-enhanced Raman scattering (SERS). MIP grafted optosensing materials, namely quantum dot (QD) composites, even allow for fluorescence detection of non-fluorescing analytes. Nevertheless, MIP NPs are also suitable for bio-applications such as drug delivery and pseudoimmunoassay. Finally, we critically discuss limitations and applications specific problems.

Spin properties of very shallow nitrogen vacancy defects in diamond

We investigate spin and optical properties of individual nitrogen-vacancy centers located within 1-10 nm from the diamond surface. We observe stable defects with a characteristic optically detected magnetic resonance spectrum down to lowest depth. We also find a small, but systematic spectral broadening for defects shallower than about 2 nm. This broadening is consistent with the presence of a surface paramagnetic impurity layer [Tisler et al., ACS Nano 3, 1959 (2009)] largely decoupled by motional averaging. The observation of stable and well-behaved defects very close to the surface is critical for single-spin sensors and devices requiring nanometer proximity to the target.

Particle sorting using a porous membrane in a microfluidic device

Porous membranes have been fabricated based on the development of the perforated membrane mold [Y. Luo and R. N. Zare, Lab Chip, 2008, 8, 1688-1694] to create a single filter that contains multiple pore sizes ranging from 6.4 to 16.6 µm inside a monolithic three-dimensional poly(dimethylsiloxane) microfluidic structure. By overlapping two filters we are able to achieve smaller pore size openings (2.5 to 3.3 µm). This filter operates without any detectable irreversible clogging, which is achieved using a cross-flow placed in front of each filtration section. The utility of a particle-sorting device that contains this filter is demonstrated by separating polystyrene beads of different diameters with an efficiency greater than 99.9%. Additionally, we demonstrate the effectiveness of this particle-sorting device by separating whole blood samples into white blood cells and red blood cells with platelets.

Natural and biomimetic materials for the detection of insulin.

Microgravimetric sensors have been developed for detection of insulin by using quartz crystal microbalances as transducers, in combination with sensitive layers. Natural antibodies as coatings were compared with biomimetic materials to fabricate mass-sensitive sensors. For this purpose polyurethane was surface imprinted by insulin, which acts as a synthetic receptor for reversible analyte inclusion. The sensor responses for insulin give a pronounced concentration dependence, with a detection limit down to 1 μg/mL and below. Selectivity studies reveal that these structured polymers lead to differentiation between insulin and glargine. Moreover, antibody replicae were generated by a double imprinting process. Thus, biological recognition capabilities of immunoglobulins are transferred to synthetic polymers. In the first step, natural-immunoglobulin-imprinted nanoparticles were synthesized. Subsequently, these templated particles were utilized for creating positive images of natural antibodies on polymer layers. These synthetic coatings, which are more robust than natural analogues, can be produced in large amount. These biomimetic sensors are useful in the biotechnology of insulin monitoring.

Detection of viruses with molecularly imprinted polymers integrated on a microfluidic biochip using contact-less dielectric microsensors

Rapid detection of viral contamination remains a pressing issue in various fields related to human health including clinical diagnostics, the monitoring of food-borne pathogens, the detection of biological warfare agents as well as in viral clearance studies for biopharmaceutical products. The majority of currently available assays for virus detection are expensive, time-consuming, and labor-intensive. In the present work we report the creation of a novel micro total analysis system (microTAS) capable of continuously monitoring viral contamination with high sensitivity and selectivity. The specific interaction between shape and surface chemistry between molecular imprinted polymer (MIP) and virus resulted in the elimination of non-specific interaction in the present sensor configuration. The additional integration of the blank (non-imprinted) polymer further allowed for the identification of non-specific adsorption events. The novel combination of microfluidics containing integrated native polymer and MIP with contact-less dielectric microsensors is evaluated using the Tobacco Mosaic Virus (TMV) and the Human Rhinovirus serotype 2 (HRV2). Results show that viral binding and dissociation events can be readily detected using contact-less bioimpedance spectroscopy optimized for specific frequencies. In the present study optimum sensor performance was achieved at 203 kHz within the applied frequency range of 5-500 kHz. Complete removal of the virus from the MIP and device reusability is successfully demonstrated following a 50-fold increase in fluid velocity. Evaluation of the microfluidic biochip revealed that microchip technology is ideally suited to detect a broader range of viral contaminations with high sensitivity by selectively adjusting microfluidic conditions, sensor geometries and choice of MIP polymeric material.

Advanced vapor recognition materials for selective and fast responsive surface acoustic wave sensors: A review

The necessity of selectively detecting various organic vapors is primitive not only with respect to regular environmental and industrial hazard monitoring, but also in detecting explosives to combat terrorism and for defense applications. Today, the huge arsenal of micro-sensors has revolutionized the traditional methods of analysis by, e.g. replacing expensive laboratory equipment, and has made the remote screening of atmospheric threats possible. Surface acoustic wave (SAW) sensors - based on piezoelectric crystal resonators - are extremely sensitive to even very small perturbations in the external atmosphere, because the energy associated with the acoustic waves is confined to the crystal surface. Combined with suitably designed molecular recognition materials SAW devices could develop into highly selective and fast responsive miniaturized sensors, which are capable of continuously monitoring a specific organic gas, preferably in the sub-ppm regime. For this purpose, different types of recognition layers ranging from nanostructured metal oxides and carbons to pristine or molecularly imprinted polymers and self-assembled monolayers have been applied in the past decade. We present a critical review of the recent developments in nano- and micro-engineered synthetic recognition materials predominantly used for SAW-based organic vapor sensors. Besides highlighting their potential to realize real-time vapor sensing, their limitations and future perspectives are also discussed.

Chemosensors for Viruses Based on Artificial Immunoglobulin Copies

2010 WILEY-VCH Verlag Gmb The increasing importance of biological analytes in chemistry has triggered the development of a vast number of techniques for rapid assessment that require materials with highly selective recognition properties. Being derived from nature, selforganization has proven a very powerful tool for actually achieving artificial receptors. Molecularly imprinted polymers (MIP) are one way to implement this into material design. They can, for example, be used as stationary phases for chromatography or in sensitive-materials’ incorporation, thereby selectively detecting a wide variety of analytes, including an early approach to target bacteria withMIP particles. Furthermore, the rugged polymeric matrix also makes it possible to determine analytes in complexmatrices, such as blood or crude plant sap, which is especially relevant for bioanalytical tasks. More recently, this has included artificial receptors for cells or plant viruses. Furthermore, Mosbach and co-workers reported on molecularly imprinted polymers to favor the formation of enzyme inhibitors. Generally speaking, the goals and strategies of molecular imprinting are quite similar to those that the human body applies for selecting specific antibodies, for example, against viruses: natural immunoglobulins (IgG) have highly variable recognition areas on the end of the shorter two arms of the Y-shaped structure. Then, the molecule optimally targeting a specific pathogen is amplified and thus ready for the respective immune response; thus, high selectivity towards a specified antigen is generated. In imprinting, on the other hand, the crucial point is to precisely mimic the chemical structure of the template on the molecular range by self-organizing the polymer chains around it. Native immunoglobulins can also be applied as selective sensor materials in analysis. However, the substantial advantages of their artificial counterparts are their mechanical and chemical robustness and their production by self-assembling processes without time-consuming, complex synthesis. Additionally, the monomeric building blocks used are often readily available by mass production. In contrast to such artificial antibodies, natural ones have to be produced and extracted from living organisms, which makes them quite expensive and tedious to obtain. Being inanimate materials, the polymeric antibodies are also resistant to chemical changes within their environment. This is not the case for biological systems, where mutation or denaturing can alter the composition of important binding sites. Within the present work, we chose to further extend the concept of molecular imprinting by not only templating a polymer with immunoglobulins (i.e., natural antibodies), but also using these MIP as stencils for designing actual plastic replicas of the initial antibody. Figure 1 sketches the concept underlying the synthesis: after first generating the MIP nanoparticle template with the respective antibody, we applied it as template in a surface imprinting process and thus generated a structured polymer surface directly on a 10MHz quartz crystal microbalance (QCM). We chose these transducers, because their mass sensitivity allows for direct, label-free detection of the respective recognition phenomena. The imprinted nanoparticles were produced by pre-polymerizing a suitable monomer solution in the presence of the antibody and precipitating them. For this purpose, we transferred the monomer solution into vigorously stirred acetonitrile, which is a poor solvent for the polymer and thus leads to particle formation. Reference particles were generated by the same synthetic pathway, but without adding the template immunoglobulin. All particles consisted of poly(vinylpyrrolidoneco-methacrylic acid) crosslinked with N,N’-(1,2-dihydroxyethylene)bisacrylamide (DHEBA). Systematic assessment of the polymerization reaction revealed that higher amounts of crosslinker favor precipitation of the respective particles. Furthermore, the size of the particles can be varied by pre-polymerizing for different amounts of time or modifying the amount of pre-polymer injected into the acetonitrile. After precipitation, we coated microscope slides with the particles and thus generated adhered layers that are then suitable templates for further imprinting.

Investigation of Surface Magnetic Noise by Shallow Spins in Diamond

T. Rosskopf, A. Dussaux, K. Ohashi, M. Loretz, R. Schirhagl, H. Watanabe, S. Shikata, K. M. Itoh, and C. L. Degen1∗ Department of Physics, ETH Zurich, Schafmattstrasse 16, 8093 Zurich, Switzerland. School of Fundamental Science and Technology, Keio University, Yokohama 223-8522, Japan. Correlated Electronics Group, Electronics and Photonics Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba Central 4, 1-1-1, Higashi, Tsukuba, Ibaraki 305-8562, Japan. Diamond Research Group, Research Institute for Ubiquitous Energy Devices, National Institute of Advanced Industrial Science and Technology (AIST), 1-8-31, Midorigaoka, Ikeda, Osaka 563-8577, Japan. (Dated: May 15, 2014)

Separation of bacteria with imprinted polymeric films.

Separation of compounds out of complex mixtures is a key issue that has been solved for small molecules by chromatography. However, general methods for the separation of large bio-particles, such as cells, are still challenging. We demonstrate integration of imprinted polymeric films (IPF) into a microfluidic chip, which preferentially capture cells matching an imprint template, and separate strains of cyanobacteria with 80-90% efficiency, despite a minimal difference in morphology and fluorescence, demonstrating its general nature. It is currently thought that the imprinting process, conducted while the polymer cures, transfers chemical information of the cells external structure to the substrate. Capture specificity and separation can be further enhanced by orienting the imprints parallel to the flow vector and tuning the pH to a lower range.