Saher Maswadi

Probe beam deflection technique as acoustic emission directionality sensor with photoacoustic emission source.

The goal of this paper is to demonstrate the unique capability of measuring the vector or angular information of propagating acoustic waves using an optical sensor. Acoustic waves were generated using photoacoustic interaction and detected by the probe beam deflection technique. Experiments and simulations were performed to study the interaction of acoustic emissions with an optical sensor in a coupling medium. The simulated results predict the probe beam and wavefront interaction and produced simulated signals that are verified by experiment.

Photoacoustic emission from fluorescent nanodiamonds enhanced with gold nanoparticles

Fluorescent nanodiamonds (FNDs) have drawn much attention in recent years for biomedical imaging applications due to their desired physical properties including excellent photostability, high biocompatibility, extended far-red fluorescence emission, and ease of surface functionalization. Here we explore a new feature of FNDs, i.e. their photoacoustic emission capability, which may lead to potential applications of using FNDs as a dual imaging contrast agent for combined fluorescence and photoacoustic imaging modalities. We observed significant enhancement of photoacoustic emission from FNDs when they were conjugated with gold nanoparticles (GNPs).

Characterization of pressure transients generated by nanosecond electrical pulse (nsEP) exposure

The mechanism(s) responsible for the breakdown (nanoporation) of cell plasma membranes after nanosecond pulse (nsEP) exposure remains poorly understood. Current theories focus exclusively on the electrical field, citing electrostriction, water dipole alignment and/or electrodeformation as the primary mechanisms for pore formation. However, the delivery of a high-voltage nsEP to cells by tungsten electrodes creates a multitude of biophysical phenomena, including electrohydraulic cavitation, electrochemical interactions, thermoelastic expansion, and others. To date, very limited research has investigated non-electric phenomena occurring during nsEP exposures and their potential effect on cell nanoporation. Of primary interest is the production of acoustic shock waves during nsEP exposure, as it is known that acoustic shock waves can cause membrane poration (sonoporation). Based on these observations, our group characterized the acoustic pressure transients generated by nsEP and determined if such transients played any role in nanoporation. In this paper, we show that nsEP exposures, equivalent to those used in cellular studies, are capable of generating high-frequency (2.5 MHz), high-intensity (>13 kPa) pressure transients. Using confocal microscopy to measure cell uptake of YO-PRO®-1 (indicator of nanoporation of the plasma membrane) and changing the electrode geometry, we determined that acoustic waves alone are not responsible for poration of the membrane.

All-optical optoacoustic microscopy based on probe beam deflection technique

Optoacoustic (OA) microscopy using an all-optical system based on the probe beam deflection technique (PBDT) for detection of laser-induced acoustic signals was investigated as an alternative to conventional piezoelectric transducers. PBDT provides a number of advantages for OA microscopy including (i) efficient coupling of laser excitation energy to the samples being imaged through the probing laser beam, (ii) undistorted coupling of acoustic waves to the detector without the need for separation of the optical and acoustic paths, (iii) high sensitivity and (iv) ultrawide bandwidth. Because of the unimpeded optical path in PBDT, diffraction-limited lateral resolution can be readily achieved. The sensitivity of the current PBDT sensor of 22 μV/Pa and its noise equivalent pressure (NEP) of 11.4 Pa are comparable with these parameters of the optical micro-ring resonator and commercial piezoelectric ultrasonic transducers. Benefits of the present prototype OA microscope were demonstrated by successfully resolving micron-size details in histological sections of cardiac muscle.

Optoacoustic multispectral imaging of radiolucent foreign bodies in tissue.

Optoacoustic imaging is an emerging medical technology that uniquely combines the absorption contrast of optical imaging and the penetration depth of ultrasound. While it is not currently employed as a clinical imaging modality, the results of current research strongly support the use of optoacoustic-based methods in medical imaging. One such application is the diagnosis of the presence of soft tissue foreign bodies. Because many radiolucent foreign bodies have sufficient contrast for imaging in the optical domain, laser-induced optoacoustic imaging could be advantageous for the detection of such objects. Common foreign bodies have been scanned over a range of visible and near infrared wavelengths by using an optoacoustic method to obtain the spectroscopic properties of the materials commonly associated with these foreign bodies. The derived optical absorption spectra compared quite closely to the absorption spectra generated when using a conventional spectrophotometer. By using the probe-beam deflection technique, a novel, pressure-wave detection method, we successfully generated optoacoustic spectroscopic plots of a wooden foreign body embedded in a tissue phantom, which closely resembled the spectrum of the same object obtained in isolation. A practical application of such spectra is to assemble a library of spectroscopic data for radiolucent materials, from which specific characteristic wavelengths can be selected for use in optimizing imaging instrumentation and provide a basis for the identification of the material properties of particular foreign bodies.

Detection of gold-nanorod targeted pathogens using optical and piezoelectric optoacoustic sensors: comparative study

We demonstrated the ability to detect surface antigens, associated with pathogens, utilizing laser optoacoustic spectroscopy with antibody-coupled gold nanorods (GNR) as a contrast agent specifically targeted to the antigen of interest. The sensitivity of the technique has been assessed by determining the minimum detectable concentration of a surface antigen from biological samples. We compared the sensitivity and applicability of two different methods for detecting optoacoustic responses, using either optical or piezoelectric sensors. Biological samples were adsorbed to the inside walls of detachable, flat-bottomed plastic micro-wells, and then probed with appropriate antibodies conjugated with gold nanorods. If the target antigens were present, the antibody-nanoparticle conjugates were bound, while any nonadsorbed nanoparticles were washed out of the wells. Optoacoustic signals were generated from the bound nanorods using a pulsed pump laser at wavelengths corresponding to one of the peak absorptions of the nanorods. Optoacoustic responses were obtained from the samples using both detection modalities. The sensitivity, suitability, ease of use of each method were assessed and compared. Initial results indicate that optical detection gives comparable sensitivity as the piezoelectric method, and further enhancement of the detection sensitivity is possible for both methods. An advantage of the piezoelectric detection method is that it may be implemented in a more compact assembly, compared to the optical method, however, the optical method may be less sensitive to external electromagnetic and acoustic noise. Further evaluation will be required to refine these measurements. The results with both methods indicate that the use of antibody-targeted nanorod contrast agents, with laser-optoacoustic detection, is a promising technology for the development of rapid in vitro diagnostic tests.

OPTOACOUSTIC SENSING OF OCULAR BACTERIAL ANTIGEN USING TARGETED GOLD NANORODS

Bacterial contamination can be detected using a minimally invasive optical method, based on laser-induced optoacoustic spectroscopy, to probe for specific antigens associated with a specific infectious agent. As a model system, we have used a surface antigen (Ag), isolated from Chlamydia trachomatis, and a complementary antibody (Ab). A preparation of 0.2 mg/ml of monoclonal Ab specific to the C. trachomatis surface Ag was conjugated to gold nanorods using standard commercial reagents, in order to produce a targeted contrast agent with a strong optoacoustic signal. The C. trachomatis Ag was absorbed in standard plastic microwells, and the binding of the complementary Ab-nanorod conjugate was tested in an immunoaffinity assay. Optoacoustic signals were elicited from the bound nanorods, using an optical parametric oscillator (OPO) laser system as the optical pump. The wavelength tuneability of the OPO optimized the spectroscopic measurement by exciting the nanorods at their optical absorption maxima. Optoacoustic responses were measured in the microwells using a probe beam deflection technique. Immunoaffinity assays were performed on several dilutions of purified C. trachomatis antigen ranging from 50 μg/ml to 1 pg/ml, in order to determine the detection limit for the optoacoustic-based assay. Only when the antigen was present, and the complementary Ab-NR reagent was introduced into the microwell, was an enhanced optoacoustic signal obtained, which indicated specific binding of the Ab-NR complex. The limit of detection with the current system design is between 1 and 5 pg/ml of bacterial Ag.

Optoacoustic sensor for nanoparticle linked immunosorbent assay (NanoLISA)

We developed an optoacoustic biosensor intended for the detection of bloodborne microorganisms using immunoaffinity reactions of antibody-coupled gold nanorods as contrast agents specifically targeted to the antigen of interest. Optoacoustic responses generated by the samples are detected using a wide band ultrasonic transducer. The sensitivity of the technique has been assessed by determining minimally detectable optical density which corresponds to the minimum detectable concentration of the target viral surface antigens. Both ionic solutions and gold nanorods served as the contrast agent generating the optoacoustic response. The sensitivity of Nano-LISA is at least OD=10-6 which allows reliable detection of 1 pg/ml (depending on the commercial antibodies that are used). Adequate detection sensitivity, as well as lack of non-specific cross-reaction between antigens favors NanoLISA as a viable technology for biosensor development.

Investigational detection of pharmacological agents in the eye using photoacoustic spectroscopy

This research reports progress in our earlier investigation of detecting specific drug diffusion into eye tissue using photoacoustic spectroscopy (PAS). A key improvement to the technique is using short pulse tunable laser source to stimulate the photoacoustic effect in tissue. An optical parametric oscillator (OPO) laser system was used as a pumping source to generate ultrasonic photoacoustic signals and employed to scan through different wavelengths with 0.1nm wavelength resolution to determine spectra of different drug solutions in an ocular phantom. The short pulse duration (5-10ns) of the OPO laser has significantly increased the photoacoustic efficiency conversion, and the ability to tune its output from 210nm to1800nm has provided a wide selection range that is useful for optimizing spectroscopic studies. PAS spectra of different solutions of molecules, such as Trypan Blue, Rose Bengal, Indocyanine Green (ICG), and Amphotericin B (AB), at concentrations as low as 1 &mgr;g/ml, were constructed and compared to their actual optical absorption spectra. Ultrasonic hydrophone and photothermal deflection technique (PhDT), a noncontact optical method, were both used to record the photoacoustic signals, and compared in terms of sensitivity and applicability to record signals from the ocular tissue-bearing phantom. The results show good agreement between the optical and photoacoustic spectra, which supports moving to an in vivo application of recording the PAS responses from the eye. Future work will be directed at adapting this method for in vivo measurements, as well as improve the data acquisition system for faster PAS signal analysis.

Investigation of photoacoustic spectroscopy for biomolecular detection

We are developing a non- or minimally-invasive method for detecting and measuring specific drugs and biomolecules in vivo using photoacoustic spectroscopy (PAS). This pilot study investigated the feasibility of detecting the concentration of certain drugs in the vitreous or aqueous of the eye. As a prototype for using PAS for molecular detection in vivo, the technique was applied to the detection in a surrogate eye, of drugs with known optical spectrum such as Trypan Blue, Rose Bengal, and Amphotericin B (AB), at concentrations as low as 1 μg/ml. Chopped CW, or short pulse, Q-switch lasers, were used as pumping sources to generate ultrasonic photoacoustic signals in an ocular phantom containing the drug solutions. In addition to an ultrasonic hydrophone, the photothermal deflection technique (PhDT), a non-contact optical method with high sensitivity and fast response, were used to record the photoacoustic signals. The data from both detectors were compared over a range of drug concentrations. The photoacoustic signal generated from the retina was used as a reference, to measure the attenuation of light through drug solutions of different concentrations in the ocular phantom. The results indicated that photoacoustic spectroscopy is feasible in ocular phantoms incorporating ex vivo ocular tissue. The signals recorded using PAS were to be found to be linearly dependent on drug concentration, as predicted by theory. The photoacoustic method was found to be sensitive to drug concentrations as low as 1 μg/ml, a clinically relevant concentration for many drugs. Future work will be directed at adapting this method for in vivo measurement, and enhancing its sensitivity by using a tunable laser as the pump source.