Extraordinary optical transmission brightens near-field fiber probe

Near-field scanning optical microscopy (NSOM) offers high optical resolution beyond the diffraction limit for various applications in imaging, sensing, and lithography; however, for many applications the very low brightness of NSOM aperture probes is a major constraint. Here, we report a novel NSOM aperture probe that gives a 100× higher throughput and 40× increased damage threshold than conventional near-field aperture probes. These brighter probes facilitate near-field imaging of single molecules with apertures as small as 45 nm in diameter. We achieve this improvement by nanostructuring the probe and by employing a novel variant of extraordinary optical transmission, relying solely on a single aperture and a coupled waveguide. Comprehensive electromagnetic simulations show good agreement with the measured transmission spectra. Due to their significantly increased throughput and damage threshold, these resonant configuration probes provide an important step forward for near-field applications.     

In vivo optical imaging and dynamic contrast methods for biomedical research

This paper provides an overview of optical imaging methods commonly applied to basic research applications. Optical imaging is well suited for non-clinical use, since it can exploit an enormous range of endogenous and exogenous forms of contrast that provide information about the structure and function of tissues ranging from single cells to entire organisms. An additional benefit of optical imaging that is often under-exploited is its ability to acquire data at high speeds; a feature that enables it to not only observe static distributions of contrast, but to probe and characterize dynamic events related to physiology, disease progression and acute interventions in real time. The benefits and limitations of in vivo optical imaging for biomedical research applications are described, followed by a perspective on future applications of optical imaging for basic research centred on a recently introduced real-time imaging technique called dynamic contrast-enhanced small animal molecular imaging (DyCE).     

Surface charge sensitivity of silicon nanowires: size dependence

Silicon nanowires of different widths were fabricated in silicon on insulator (SOI) material using conventional process technology combined with electron-beam lithography. The aim was to analyze the size dependence of the sensitivity of such nanowires for biomolecule detection and for other sensor applications. Results from electrical characterization of the nanowires show a threshold voltage increasing with decreasing width. When immersed in an acidic buffer solution, smaller nanowires exhibit large conductance changes while larger wires remain unaffected. This behavior is also reflected in detected threshold shifts between buffer solutions of different pH, and we find that nanowires of width >150 nm are virtually insensitive to the buffer pH. The increased sensitivity for smaller sizes is ascribed to the larger surface/volume ratio for smaller wires exposing the channel to a more effective control by the local environment, similar to a surrounded gate transistor structure. Computer simulations confirm this behavior and show that sensing can be extended even down to the single charge level.     

Chemical sensing plate with a laser-terahertz monitoring system

A new type of laser-terahertz emission system for noncontact investigations of chemical solutions has been developed. The system monitors terahertz emission from a sensing plate, which consists of silicon oxide and silicon thin film layers on a sapphire substrate. Sensing of chemical solutions with pH values between 1.68 and 10.01 was demonstrated. The amplitude of the terahertz emission from the sensing plate increased with increasing pH value. This change in the amplitude was caused by a change in the depletion layers of the silicon thin film when protons were adsorbed on the surface of the sensing plate. This study demonstrates that full noncontact monitoring of chemical solutions is possible using the laser-terahertz emission system.     

Optical Replacement of pH Electrode

The development of a noninvasive, self-contained optical pH sensor probe intended as a direct replacement for a pH electrode is presented. It uses a fluorescent excitation-ratiometric pH sensing dye in a patch. The patch is excited by light emitting diodes of differing wavelengths which are controlled by a microcontroller (MCU). The emission levels are measured by analog circuitry and their ratio is converted by the MCU into a linear output which mirrors that of a conventional pH electrode. The optical sensor readings were consistent with readings from a glass pH electrode with average difference .061 pH. This sensor allows noninvasive, low-cost optical pH sensing with the ability to interface with existing pH monitoring equipment.      

Temperature-Dependent Refractive Index Determination from Critical Angle Measurements: Implications for Quantitative SPR Sensing

Temperature-dependent measurements of surface coverage and interfacial kinetics remain relatively unexploited in thin-film sensing applications that rely on optical surface-sensitive techniques such as surface plasmon resonance spectroscopy (SPR). These techniques are inherently sensitive to the optical properties of the bulk solution in contact with the thin film; therefore, quantitative thin-film sensing requires accurate refractive index data for bulk solutions at the conditions of interest. The refractive index for bulk solutions depends strongly on temperature, solution composition, and optical excitation wavelength. In this paper, we demonstrate the use of critical angle measurements for accurate, independent determination of the refractive index of bulk solutions and present results for different experimental conditions of solution temperature, solution concentration, and excitation wavelength. We also examine the implications of incorrect accounting of the bulk solution for the case of two-color SPR sensing of ultrathin organic films. This sensing technique, which depends inherently on the contrast in the dispersion of the refractive index of the film and the bulk solution, can be over 1 order of magnitude more sensitive than single-color SPR measurements. Critical angle measurements can be implemented in conjunction with SPR measurements and will be invaluable for thin film sensing application in which the bulk refractive index varies during the experiment, for example, in temperature-dependent SPR measurements, or for applications in which the solution refractive index is not known.     

光学3D坐标测量技术研究

研究了光学坐标测量系统三维精密视觉测量方法,对其核心技术:数字成像器件模型及标定方法、辅助靶标技术以及高精度亚象素图像处理算法进行了深入的讨论,并提出了详细的解决方法.经实验证明,这些方法切实可行,达到了所要求的精度,从而为光学坐标测量系统的设计提供了可靠依据.     

Silicon field effect transistors as dual-use sensor-heater hybrids

We demonstrate the temperature mediated applications of a previously proposed novel localized dielectric heating method on the surface of dual purpose silicon field effect transistor (FET) sensor-heaters and perform modeling and characterization of the underlying mechanisms. The FETs are first shown to operate as electrical sensors via sensitivity to changes in pH in ionic fluids. The same devices are then demonstrated as highly localized heaters via investigation of experimental heating profiles and comparison to simulation results. These results offer further insight into the heating mechanism and help determine the spatial resolution of the technique. Two important biosensor platform applications spanning different temperature ranges are then demonstrated: a localized heat-mediated DNA exchange reaction and a method for dense selective functionalization of probe molecules via the heat catalyzed complete desorption and reattachment of chemical functionalization to the transistor surfaces. Our results show that the use of silicon transistors can be extended beyond electrical switching and field-effect sensing to performing localized temperature controlled chemical reactions on the transistor itself.     

Real-time in situ Raman analysis of microwave-assisted organic reactions

Integration of a Raman probe into a commercially available microwave synthesizer has demonstrated unprecedented utility in understanding chemical processes within the rapidly emerging field of microwave-assisted organic synthesis. The real-time spectral feedback afforded by this system has facilitated analysis of reaction mechanisms, reactive intermediates, and reaction kinetics via optical sampling through the sidewall of the sealed reaction vial within the microwave chamber. The feasibility, attributes, and limitations of the system are illustrated using amine coupling and Knoevenagel coupling example reactions. In addition to the reported analyses, this system provided the safety of remote sensing, adequate sensitivity, ease of alignment, and optimized "depth of field" for analysis of solutions with solids content.     

Carbon nanotube scanning probe for imaging in aqueous environment

Carbon nanotubes (CNTs) used as a probe for scanning probe microscopy has become one of the many potential usages of CNTs that is finding real applications in scientific research and industrial communities. It has been proposed that the unique mechanical buckling properties of the CNT would lessen the imaging force exerted on the sample and, thus, make CNT scanning probes ideal for imaging soft materials, including biological samples in liquid environments. The hydrophobic nature of the CNT graphitic sidewall is clearly chemically incompatible with the aqueous solution requirements in some biological imaging applications. In this paper, we present electron micrograph results demonstrating the instability of CNT scanning probes when submerged in aqueous solution. Moreover, we also introduce a novel approach to resolve this chemical incompatibility problem. By coating the CNT probe with ethylenediamine, thus rendering the CNT probe less hydrophobic, we demonstrate the liquid imaging capability of treated CNT probes. Experimental data for imaging in aqueous solutions are presented, which include an ultrathin Ir film and DNA molecules on a mica surface.     

pH sensitive multiwalled carbon nanotubes

The CNTs-based sensors have received considerable attention because of their outstanding properties, such as faster response, higher sensitivity, and lower operating temperature. And we expect that CNTs-based electrochemical sensors offer substantial improvements in the performance of pH sensing device. This letter reports experimental results that demonstrate the pH sensing capability of the multiwalled carbon nanotubes (MWCNTs) film by using the thermal chemical vapor deposition (thermal CVD). It was found that electronic properties of MWCNTs can be changed by the introduction of different pH value solutions. The absorption of the hydroxide in pH buffer solution changes conductivity of the MWCNTs. We observed in situ measurement of electrical conductivity by cycling solution range from acid to base.     

Thermal Characterization of Solutions Containing Gold Nanoparticles at Different pH Values

The thermal lens spectrometry (TLS) technique was used to obtain the thermal diffusivity of solutions containing gold nanoparticles (15 nm average diameter) at different pH values. TLS, in a mode-mismatched dual beam configuration, provides a reliable alternative to measure, with high sensitivity, the thermal diffusivities of semitransparent materials, and low thermal diffusivities. The results show that the nanofluid thermal diffusivity increases when the pH is increased. These results will be compared with reported studies for nanofluids with variable pH. From this comparison, it can be seen that the pH values of the solutions influenced the superficial density of charges in the nanoparticles. Also, optical absorption spectra for these gold nanoparticle solutions were obtained using a spectrophotometer, and the nanoparticle size was obtained by the TEM technique. The present measurements were performed at room temperature. This study is important for some medical applications such as photothermal cancer therapy.     

3,3',5,5'-tetramethyl-N-(9-anthrylmethyl)benzidine: a dual-signaling fluorescent reagent for optical sensing of aliphatic aldehydes

A new optical chemical sensor for continuous monitoring of aliphatic aldehydes has been proposed based on the reversible chemical reaction between a new sensing reagent, 3,3',5,5'-tetramethyl-N-(9-anthrylmethyl)benzidine (TMAB), and the analytes. TMAB, containing two receptors and two fluorescent reporters, can perform dual fluorescence responses corresponding to the reactions of hydrogen ion and carbonyl compound. When immobilized in a plasticized poly(vinyl chloride) membrane, TMAB extracts aliphatic aldehydes from aqueous solution into the bulk membrane phase and reacts with the analyte by forming a Schiff base. Since the extraction equilibrium and chemical reaction are accompanied by fluorescence increase of the sensing membrane, the chemical recognition process could be directly translated into an optical signal. At pH 3.20, the sensor exhibits a dynamic detection range from 0.017 to 4.2 mM n-butyraldehyde with a limit of detection of 0.003 mM. The forward response time (t95) of the sensor is 3-5 min, and the reverse response time is 5-7 min. The responses of the sensor toward different kinds of aldehydes and ketones depend on the lipophilicity and the reactivity of the analytes. Since the fluorescence enhancement of the sensing membrane at 296 nm/410 nm is only related to the formation of Schiff base, the measurement of aldehydes is independent of pH.     

Fast Scanning Probe Microscopy via Machine Learning: Non‐Rectangular Scans with Compressed Sensing and Gaussian Process Optimization

Fast scanning probe microscopy enabled via machine learning allows for a broad range of nanoscale, temporally resolved physics to be uncovered. However, such examples for functional imaging are few in number. Here, using piezoresponse force microscopy (PFM) as a model application, a factor of 5.8 reduction in data collection using a combination of sparse spiral scanning with compressive sensing and Gaussian process regression reconstruction is demonstrated. It is found that even extremely sparse spiral scans offer strong reconstructions with less than 6% error for Gaussian process regression reconstructions. Further, the error associated with each reconstructive technique per reconstruction iteration is analyzed, finding the error is similar past ≈15 iterations, while at initial iterations Gaussian process regression outperforms compressive sensing. This study highlights the capabilities of reconstruction techniques when applied to sparse data, particularly sparse spiral PFM scans, with broad applications in scanning probe and electron microscopies.     

Nanoparticle layer deposition for plasmonic tuning of microstructured optical fibers

Plasmonic nanoparticles with spectral properties in the UV-to-near-IR range have a large potential for the development of innovative optical devices. Similarly, microstructured optical fibers (MOFs) represent a promising platform technology for fully integrated, next-generation plasmonic devices; therefore, the combination of MOFs and plasmonic nanoparticles would open the way for novel applications, especially in sensing applications. In this Full Paper, a cost-effective, innovative nanoparticle layer deposition (NLD) technique is demonstrated for the preparation of well-defined plasmonic layers of selected particles inside the channels of MOFs. This dynamic chemical deposition method utilizes a combination of microfluidics and self-assembled monolayer (SAM) techniques, leading to a longitudinal homogeneous particle density as long as several meters. By using particles with predefined plasmonic properties, such as the resonance wavelength, fibers with particle-adequate spectral characteristics can be prepared. The application of such fibers for refractive-index sensing yields a sensitivity of about 78 nm per refractive index unit (RIU). These novel, plasmonically tuned optical fibers with freely selected, application-tailored optical properties present extensive possibilities for applications in localized surface plasmon resonance (LSPR) sensing.     

Three-dimensional reduced-symmetry of colloidal plasmonic nanoparticles

Owing to their novel optical properties, three-dimensional plasmonic nanostructures with reduced symmetry such as a nanocrescent and a nanocup have attracted considerable current interest in biophotonic imaging and sensing. However, their practical applications have been still limited since the colloidal synthesis of such structures that allows, in principle, for in vivo application and large-scale production has not been explored yet. To date, these structures have been fabricated only on two-dimensional substrates using micro/nanofabrication techniques. Here we demonstrate an innovative way of breaking symmetry of colloidal plasmonic nanoparticles. Our strategy exploits the direct overgrowth of Au on a hybrid colloidal dimer consisting of Au and polystyrene (PS) nanoparticles without the self-nucleation of Au in an aqueous solution. Upon the overgrowth reaction, the steric crowding of PS leads to morphological evolution of the Au part in the dimer ranging from half-shell, nanocrescent to nanoshell associated with the appearance of the second plasmon absorption band in near IR. Surface-enhanced Raman scattering signal is obtained directly from the symmetry-broken nanoparticles solution as an example showing the viability of the present approach. We believe our concept represents an important step toward a wide range of biophotonic applications for optical nanoplasmonics such as targeting, sensing/imaging, gene delivery, and optical gene regulations.     

Combined scanning electrochemical/optical microscopy with shear force and current feedback

A technique that combines scanning electrochemical microscopy (SECM) and scanning optical microscopy (OM) was developed. Simultaneous scanning electrochemical/optical microscopy (SECM/OM) was performed by a special probe tip, which consists of an optical fiber core for light passage, surrounded by a gold ring electrode, and an outermost electrophoretic insulating sheath, with the tip attached to a tuning fork. To regulate the tip-substrate distance, either the shear force or the SECM tip current was employed as the feedback signal. The application of a quartz crystal tuning fork (32.768 kHz) for sensing shear force allowed simultaneous topographic, along with SECM and optical imaging in a constant-force mode. The capability of this technique was confirmed by obtaining simultaneously, for the first time, topographic, electrochemical, and optical images of an interdigitated array electrode. Current feedback from SECM also provided simultaneous electrochemical and optical images of relatively soft samples, such as a polycarbonate membrane filter and living diatoms in a constant-current mode. This mode should be useful in mapping the biochemical activity of a living cell.     

光纤传感器在生物医学中的应用进展

光纤在医学和生物学中得到了广泛的应用,从光管道和压力传感器到复杂的化学传感器都与光纤有关.相干光纤束可用于内窥镜成像,而单光纤可用于近红外分层成像和光学相干分层成像.采用光纤还能方便地将光辐射传输到组织内,以激活靶标化学治疗药物.利用平面光纤光导将光波传输到测定部位的化学传感技术可以进行光度和荧光分析.光纤化学传感器还具有表面分子识别位点或化学反应部位,可用于特定分子的检测.这些化学传感器基于表面等离子体共振、干涉、光谱测量或荧光测量等原理.酶的生物识别或抗原抗体结合使光纤传感器可以获得高的特异性.近年来,测定的靶标分子的范围已从简单的气体分子和离子发展到了DNA等大分子.     

Determination of swelling of responsive gels with nanometer resolution. Fiber-optic based platform for hydrogels as signal transducers

A novel technique for detection of hydrogel swelling intended for use as a chemical or biological sensor, but also generally applicable for obtaining high-precision hydrogel swelling data, is described. The underlying design principle is that a hydrogel bound to the tip of an optical fiber constituting the environmental sensing element makes up a Fabry-Perot cavity for high-resolution detection of the optical length. The interference of light guided by the optical fiber and reflected at the two interfaces, fiber-gel and gel-solution, enables optical detection of the optical path length within the gel and degree of swelling of the gel. Acrylamide-based hydrogels with various molar fractions of the cationic monomer, N-(3-dimethylaminopropyl)acrylamide, were fabricated at the end of the fiber to demonstrate the feasibility of the approach. These sensors were investigated in solutions of varying ionic strength and pH. Relative gel length changes of the approximately 50-microm half-spherical gels were determined with a precision of approximately 2 nm. Moreover, the combination of good reproducibility and resolution of determination of swelling supports measurements of ionic strength changes in the millimolar range. Kinetic measurements for gel swelling induced by changes in ionic strengths had a time constant of approximately 2 s (half-spherical gel with 60-microm radius), whereas the time constants for gel swelling induced by changes in pH were observed in the range 90-130 s. Thus, different processes dictate the swelling rate in the two different cases. The results show that hydrogel equilibrium swelling and kinetics can be determined by the optical interference method with nanometer resolution, thus providing a unique platform for characterization of hydrogels swelling in general, and using functionalized hydrogels as biological sensors in particular.