265 kHz Research Articles

Piezoceramic membrane with built-in ultrasound for reactive oxygen species generation and synergistic vibration anti-fouling

Piezoceramic membranes have emerged as a prominent solution for membrane fouling control. However, the prevalent use of toxic lead and limitations of vibration-based anti-fouling mechanism impede their wider adoption in water treatment. This study introduces a Mn/BaTiO3 piezoceramic membrane, demonstrating a promising in-situ anti-fouling efficacy and mechanism insights. When applied to an Alternating Current at a resonant frequency of 20 V, 265 kHz, the membrane achieves optimal vibration, effectively mitigating various foulants such as high-concentration oil (2500 ppm, including real industrial oil wastewater), bacteria and different charged inorganic colloidal particles, showing advantages over other reported piezoceramic membranes. Importantly, our findings suggest that the built-in ultrasonic vibration of piezoceramic membranes can generate reactive oxygen species. This offers profound insights into the distinct anti-fouling processes for organic and inorganic wastewater, supplementing and unifying the traditional singular vibrational anti-fouling mechanism of piezoceramic membranes, and potentially propelling the development of piezoelectric catalytic membranes.

Fabrication of a piezoelectric micromachined ultrasonic transducer (PMUT) with dual heterogeneous piezoelectric thin film stacking

In this study, a piezoelectric micromachined ultrasonic transducer (pMUT) was developed using dual heterogeneous piezoelectric thin films stacked on top of each other. One piezoelectric layer is specialized for the actuation, while the other is specialized for the reception of the ultrasonic waves. This combined use of two materials promises to realize a pMUT transceiver array with an excellent transmitting and receiving performance and a high fill factor. Taking fabrication feasibility into consideration, AlN/Pb(Mg1/3, Nb2/3)O3-PbTiO3 (PMN-PT) and Pb(Zr, Ti)O3 (PZT)/AlN pMUTs were selected as two candidates for prototyping as the dual-layer pMUTs. The driving tests were performed by actuation of each piezoelectric layer and a resonance frequencies around 265 kHz and 203 kHz were confirmed for AlN/PMN-PT and PZT/AlN pMUT, respectively. The diaphragm of AlN/PMN-PT pMUT has a displacement sensitivity of 3538 nm V−1 and 306 nm V−1 when actuating PMN-PT layer and AlN layer at resonance frequency, respectively. While the diaphragm of PZT/AlN pMUT has a displacement sensitivity of 1036 nm V−1 and 744 nm V−1 when actuating the PZT layer and the AlN layer at the resonance frequency, respectively.

Investigation of spatio-temporal inertial cavitation activity for optimization of needle-free ultrasound-enhanced vaccine delivery

Ultrasound-induced cavitation is a promising mechanism for pain-free delivery of vaccines without needles. However, the relationships between cavitation energy and the various bioeffects involved in transdermal vaccination, including skin permeabilization, convective drug transport, reversible and irreversible sonoporation, and immune-stimulation, are not well understood. Previous transdermal ultrasound experiments have demonstrated inhomogeneous delivery across the exposed surface, which remains poorly understood. In this work, we first seek to fully characterize and quantify spatio-temporal cavitation activity across the skin surface, to identify a local cavitation dose that is optimal for all four key bioeffects. We have designed a new in vitro experimental setup that exposes several potential skin models to 265 kHz focused ultrasound, a new generation of protein-based cavitation nuclei (pCaNs), and a fluorescently labelled vaccine analogue, while simultaneously imaging cavitation activity parallel to the skin surface by Passive Acoustic Mapping (PAM). Subsequent staining and multi-photon microscopy of the skin models allows a direct comparison between the PAM-derived spatiotemporal inertial cavitation dose and locations where particular bioeffects were maximized. We will discuss the results of this investigation, how they relate to our recent in vivo study, and whether these findings enable enhancements of the spatial homogeneity and reproducibility of needle-free ultrasound vaccination.

Design of monodispersed PVP functionalized biocompatible manganese ferrite nanoparticles for hyperthermia application

Magnetic nanoparticles which can be triggered externally by an alternating current magnetic field (AMF) have shown their potential for various biomedical applications. Magnetic hyperthermia using magnetic nanoparticles (MNPs) has emerged as a promising treatment for cancer. For effective administration of MNPs in hyperthermia, they need to show high heating attributes under physiological safe values of the field, frequency, and for a minimal amount of material. In this regard, an attempt has been made to synthesize MnFe2O4 nanoparticles and functionalize them with the biocompatible polyvinylpyrrolidone (PVP) polymer. Functionalized nanoparticles are characterized for magneto-structural and morphological analysis. The heating capacity of MNPs is contemplated to evaluate the attainability to use in hyperthermia applications. Results demonstrate that MNPs exhibited superparamagnetism with high magnetization at room temperature. Maximum specific absorption rate (SAR) 468.37 W g−1 has been observed at 335.2 Oe (and frequency 265 kHz) for PVP coated nanoparticles for 5 mg mL−1 concentration. Cell viability of PVP functionalized nanoparticles performed using the L929 cell line showed that the nanoparticles are biocompatible. The PVP-functionalized MnFe2O4 nanoparticles synthesized by the one-pot polyol method can be used as a heating mediator in magnetic hyperthermia therapy.

APTES (3-aminopropyltriethoxy silane) functionalized MnFe2O4 nanoparticles: a potential material for magnetic fluid hyperthermia

Magnetic nanoparticles have the potential to be used for biomedical applications, specifically in painless curing of cancer. The primary objective of this article is to prepare 3-aminopropyltriethoxy silane (APTES) functionalized magnetic nanoparticles by using polyol synthesis method in order to use them for magnetic hyperthermia application. The obtained magnetic nanoparticles were characterized by using X-ray diffraction, scanning electron microscope, transmission electron microscopy, vibrating sample magnetometry, fourier transform infrared spectroscopy and thermogravimetric analysis techniques for structural, morphological and magnetic analysis. Structural analysis showed that the mean crystallite size of prepared nanoparticles was about 13 nm and magnetic study exhibited that the bare and functionalized nanoparticles were superparamagnetic at room temperature. Induction heating study was performed by applying external AC magnetic field of 167.6–335.2 Oe at a fixed frequency of 265 kHz to assess the feasibility for magnetic hyperthermia anticancer therapy. Maximum specific absorption rate 261.53 W g−1 has observed at 335.2 Oe (265 kHz) for APTES coated nanoparticles. Cell viability study revealed that APTES functionalized MnFe2O4 nanoparticles can be potential heating agent for cancer hyperthermia therapy as nanoparticles have almost no toxicity.

Study of Co0.5Zn0.5Fe2O4 nanoparticles for magnetic hyperthermia

The structural characteristics, magnetic properties, and processes of magnetic heating in an alternating magnetic field of magnetic nanoparticles (MNPs) Co0.5Zn0.5Fe2O4 (cobalt–zinc ferrite, CZF) are studied to explore the possibilities of their application in medicine, namely, for magnetic hyperthermia treatment (the heating of particles with external alternating magnetic field). CZF magnetic nanoparticles were obtained by coprecipitation using sodium hydroxide (NaOH) as a precipitating agent. Based on the data obtained by transmission electron microscopy in the transmission geometry, it is found that CZF magnetic nanoparticles have an almost spherical shape with an average particle size of 13 nm. X-ray diffraction and Mossbauer studies showed that CZF magnetic nanoparticles are single-phase, and their structure corresponds to a cubic spinel structure. The saturation magnetization Ms of CZF nanoparticles is measured at room temperature using a vibrating sample magnetometer. The possibility of heating CZF magnetic nanoparticles with an external alternating magnetic field was studied using an induction heating system. The specific absorption rate is determined by applying an external alternating magnetic field in the range of 167.5 to 335.2 Oe at a fixed frequency of 265 kHz. It is found that the maximum amount of heat (114.98 W/g) is produced at a concentration of 5 mg/L under a field of 335.2 Oe.

Cavitation-enhanced delivery of insulin in agar and porcine models of human skin

Ultrasound-assisted transdermal insulin delivery offers a less painful and less invasive alternative to subcutaneous insulin injections. However, ultrasound-based drug delivery, otherwise known as sonophoresis, is a highly variable phenomenon, in part dependent on cavitation. The aim of the current work is to investigate the role of cavitation in transdermal insulin delivery. Fluorescently stained, soluble Actrapid insulin was placed on the surface of human skin-mimicking materials subjected to 265 kHz, 10% duty cycle focused ultrasound. A confocally and coaxially aligned 5 MHz broadband ultrasound transducer was used to detect cavitation. Two different skin models were used. The first model, 3% agar hydrogel, was insonated with a range of pressures (0.25–1.40 MPa peak rarefactional focal pressure—PRFP), with and without cavitation nuclei embedded within the agar at a concentration of 0.05% w/v. The second, porcine skin was insonated at 1.00 and 1.40 MPa PRFP. In both models, fluorescence measurements were used to determine penetration depth and concentration of delivered insulin. Results show that in agar gel, both insulin penetration depth and concentration only increased significantly in the presence of inertial cavitation, with up to a 40% enhancement. In porcine skin the amount of fluorescent insulin was higher in the epidermis of those samples that were exposed to ultrasound compared to the control samples, but there was no significant increase in penetration distance. The results underline the importance of instigating and monitoring inertial cavitation during transdermal insulin delivery.

The influence of coating on the structural, magnetic and colloidal properties of LSMO manganite and the heating mechanism for magnetic fluid hyperthermia application

Combustion synthesized superparamagnetic La0.7Sr0.3MnO3 nanoparticles have been studied for magnetic fluid hyperthermia applications. The effect of the coating concentration of acrypol on the colloidal, magnetic and heating properties of L0.7Sr0.3MnO3 have been studied. Uncoated and acrypol coated samples were characterized by X-ray diffraction, Fourier transform infrared spectroscopy, scanning electron microscopy, transmission electron microscopy, high resolution transmission electron microscopy and vibrating sample magnetometer studies. The power absorbed by acrypol coated nanoparticles suspended in phosphate buffer solution from a 265 kHz radio frequency field was measured as a function of coating concentration and field amplitude H. For H = 40.4 kA m−1 the maximum specific absorption rate was 80.82 W g−1 for 4% coating concentration. All results evidenced that among all the coated samples, 2, 4, 6 and 8% coating concentrations, the sample with 4% coating concentration shows the highest induced heating temperature, the highest specific absorption rate and the highest biocompatibility.

Colloidal stability of polyethylene glycol functionalized Co0.5Zn0.5Fe2O4 nanoparticles: effect of pH, sample and salt concentration for hyperthermia application

The long-term colloidal stability of magnetic nanoparticles (MNPs) is an important goal when such MNPs are used for biomedical applications. Co0.5Zn0.5Fe2O4 (CZF) MNPs were functionalized with polyethylene glycol (PEG) and the effect of surface functionalization on the colloidal stability was investigated by dispersing them in double distilled water , phosphate buffer saline (PBS) and Dulbecco's modified Eagle's medium with bovine serum albumin (DMEM-BSA) for possible application in magnetic hyperthermia. The colloidal stability was studied by measuring the particle charge and hydrodynamic diameter as a function of pH and ionic strength using the dynamic light scattering (DLS) method and zeta potential measurement. DLS and zeta potential results indicated that PEG-CZF MNPs dispersed in DDW exhibited poor stability, whereas those dispersed in PBS and DMEM-BSA exhibited excellent stability. An induction heating experiment revealed that the hyperthermia temperature (42–43 °C) could be achieved by the PEG-coated sample at a magnetic field of 168–335 Oe and a fixed frequency of 265 kHz. Interestingly, a high specific absorption rate (SAR) of 193 W g−1 was observed in the case of PEG coated MNPs in PBS medium, this is a very important parameter in hyperthermia applications and such a high SAR comes from the high contrition from the Brownian rotational relaxation because of the better dispersion of MNPs in PBS medium. PEG coated MNPs at concentrations below 1.8 mg mL−1 exhibited good viability above 86% in mice fibroblast L929 cells. The results suggest that co-precipitation synthesized CZF MNPs coated with PEG can be used as potential heating agents for magnetic particle hyperthermia.

Biocompatibility, biodistribution and efficacy of magnetic nanohydrogels in inhibiting growth of tumors in experimental mice models.

We report in vivo evaluation of a thermo-responsive poly(N-isopropylacrylamide)-chitosan based magnetic nanohydrogel (MNHG) incorporated with Fe3O4 nanoparticles (NPs) in mice models with expandible scope for use in localized delivery of chemotherapeutics. Biocompatibility and biodistribution of the MNHG are studied in normal Swiss mice while efficacy in tumor growth inhibition is studied in a subcutaneous fibrosarcoma tumor. The ex vivo time-dependent pattern of accumulated MNHG into vital organs; lung, liver, spleen, kidney and brain collected at 1 h, 48 h, 7 d and 14 d post intravenous administration are investigated using both a vibrating sample magnetometer (VSM) and inductively coupled plasma-atomic emission spectroscopy (ICP-AES) method. The doses of MNHG (dose I ∼ 650 and dose II ∼ 325 μg g-1 body wt) used in the study are determined based on induced thermal activation of MNHG under an AC magnetic field (AMF). Fibrosarcoma tumor bearing mice are subjected to hyperthermia with a field of 325 Oe and 265 kHz for 30 min following intratumoral administration of dose I. Tumor size measured at an interval of 72 h for a period of 2 weeks reveals that the NPs mediated therapy decelerated the growth of the transplanted tumor by about three-fold (size, 1545 ± 720 mm3) as compared to the exponential growth of the tumor (size, 4510 ± 735 mm3) in control mice. These results suggest the feasibility of using poly(NIPAAm)-chitosan hydrogels loaded with NPs for combined thermo-chemotherapy where the efficacy may further be improved by temperature dependent release of the drugs from the magneto hydrogels.

Bent-beam sensing with triple-beam tuning forks

A piezoelectric triple-beam tuning fork (TBTF) is presented as a force sensing element. Bent-beam sensing with large deformations is introduced as an alternative to conventional in-plane sensing with small deformations. Inertial force is applied to the tuning fork by a proof mass and it is demonstrated that bent-beam sensing results in several orders of magnitude increase in force sensitivity. The findings are verified by comparing the results of a tilting experiment to an analytical solution and finite element simulations. Five mode shapes of the TBTF are investigated and the resonance frequency of the proposed sensor mode is 265 kHz.

Stability and Relaxation Mechanisms of Citric Acid Coated Magnetite Nanoparticles for Magnetic Hyperthermia

Magnetite (Fe3O4) nanoparticles are proper materials for Magnetic Fluid Hyperthermia applications whenever these conjugate stability at physiological (neutral pH) medium and high specific dissipation power. Here, magnetite nanoparticles 9–12 nm in size, electrostatically stabilized by citric acid coating, with hydrodynamic sizes in the range 17–30 nm, and well dispersed in aqueous solution were prepared using a chemical route. The influence of media acidity during the adsorption of citric acid (CA) on the suspension’s long-term stability was systematically investigated. The highest content of nanoparticles in a stable suspension at neutral pH is obtained for coating performed at pH = 4.58, corresponding to the larger amount of CA molecules adsorbed by one carboxylate link. Specific absorption rates (SARs) of various magnetite colloids, determined calorimetrically at a radio frequency field of 265 kHz and field amplitude of 40.1 kA/m, are analyzed in terms of structural and magnetic colloid properties. Larger dipolar interactions lead to larger Néel relaxation times, in some cases larger than Brown relaxation times, which in the present case enhanced magnetic radio frequency heating. The improvement of suspension stability results in a decrease of SAR values, and this decrease is even large in comparison with uncoated magnetite nanoparticles. This fact is related to interactions between particles.

Induction heating studies of dextran coated MgFe2O4nanoparticles for magnetic hyperthermia

MgFe(2)O(4) nanoparticles with sizes around 20 nm have been prepared by a combustion method and functionalized with dextran for their possible applications in magnetic particle hyperthermia. The induction heating study of these nanoparticles at different magnetic field amplitudes, from 6.7 kA m(-1) to 26.7 kA m(-1), showed self-heating temperature rise up to 50.25 °C and 73.32 °C (at 5 mg mL(-1) and 10 mg mL(-1) concentrations in water respectively) which was primarily thought to be due to hysteresis losses activated by an AC magnetic field. The dextran coated nanoparticles showed a maximum specific absorption rate (SAR) of about 85.57 W g(-1) at 26.7 kA m(-1) (265 kHz). Dextran coated nanoparticles at concentrations below 1.8 mg mL(-1) exhibit good viability above 86% on mice fibroblast L929 cells. The results suggest that combustion synthesized MgFe(2)O(4) nanoparticles coated with dextran can be used as potential heating agents in magnetic particle hyperthermia. Uncoated and dextran coated samples were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM) and vibrating sample magnetometry (VSM), Fourier transform infrared spectroscopy (FTIR), thermogravimetric-differential thermal analysis (TG-DTA) and zeta potential-DLS studies.

Seasonal and vertical variations in emergence behaviors of Neomysis americana

Emergence patterns were observed in the macrotidal Damariscotta River estuary in Maine in fall 2005 and summer 2006. High temporal and vertical resolution was achieved with two multifrequency echo sounders sampling from 265 kHz to 3 MHz. Time‐series waterfall plots and spectral analysis revealed that emergence patterns observed at 265‐420 kHz (frequencies that detect animals larger than mesozooplankton) followed combinations of diel and tidal rhythms that differed with both depth in the water column and season. The dominant emergent species at the study site, Neomysis americana, showed nocturnal emergence in summer. Toward the end of its emergence season in fall, however, its dominant rhythm shifted to semidiurnal (12.4‐h period). The timing of major emergence in the fall coincided with low slack tides near the surface but with peak flood speed near the bottom, providing plausible mechanisms of retention and selective tidal stream transport within the estuary. Our results confirm earlier suggestions from net sampling that emergence in this species is best conceived as a broadening of the vertical spread of the population distribution rather than a migration of the bulk of the population far off the bottom: Most individuals spent most of their time near the bottom, even during emergence. Prior acoustic work that did not resolve the lowermost water column reported emergent mysids to overwhelm the holoplankton in biovolume, but it, too, seriously underestimated mysid abundance because it failed to resolve this high concentration of animals in the lowermost water column. In the mesozooplankton size category, emergence reached surface waters only during low slack tides and systematically avoided times of the fastest currents. This study highlights the importance of high temporal and spatial sampling resolution in detecting and understanding components of emergence and points away from simple models of diel vertical migration toward state‐dependent, individual‐based models of habitat utilization.

Fabrication and performance of a flextensional microactuator

A flextensional microactuator has been designed and fabricated for the amplification of the small strain of the piezoelectric materials to achieve large displacements. Bulk PZT material available in the form of 500 µm thick polished substrate has been integrated with a precision micromachined silicon beam structure to achieve the clamped–clamped flextensional microactuator. A high strength, high precision (alignment) and low temperature (∼200 °C) In/Sn solder bonding process has been developed and used for the fabrication of the flextensional microactuators. Actuators with physical dimensions of the flextensional structure ranging from 350 to 600 µm in length, 50 to 100 µm in width and 5 to 6 µm in thickness were fabricated. The measured static deflection characteristics of the silicon micromachined beam show a flextensional gain factor of 20 with a large amplitude stroke of ∼8 µm when actuated using −100 V to 100 V. The bandwidth of the actuator was experimentally measured to be 265 kHz. The fabricated devices show good repeatability with a hysteresis pattern arising from the PZT characteristics. The bonding technique described here can be used for the precision integration of heterogeneous materials for MEMS device fabrication and their packaging.

Tidally phased emergence events in a strongly tidal estuary

Acoustic backscatter from an active sonar system over a range of six frequencies between 265 kHz and 3 MHz in the tidally dominated Damariscotta River estuary, Maine, United States, revealed that the major emergence event of the night commenced on the first tidal deceleration after dark (3.5-4 h after local slack), irrespective of flow direction. Emergence traps identified the mysid shrimp, Neomysis americana, as the dominant migrator. Water-column-integrated, acoustically estimated biovolume at our 10-m deep study location increased by a factor of about 6 during these large events, entirely dominating the holoplanktonic contribution and likely being a major component in benthic-pelagic coupling. Application of the same algorithm used to locate this nighttime emergence revealed a parallel but considerably smaller daytime emergence event near the same phase of the tide. Daytime trap samples failed to recover the organisms responsible, but transmissometry rejected the alternative hypothesis that we observed resuspension events. We suspect, but have yet only weak evidence, that animals emerging in daylight are copepods rather than mysids.

Light-cued emergence and re-entry events in a strongly tidal estuary

Acoustic backscatter from an active sonar system (TAPS-6) over a range of six frequencies between 265 kHz and 3 MHz revealed characteristics of emergence and reentry events within 1 h of sunset and sunrise, respectively, at a site 10 m deep in the Damariscotta River estuary, Maine. Emergence traps indicated that the mysid shrimpNeomysis americana was the dominant migrator. Daily fluctuation in irradiance influenced the timing of dusk emergence. Local variability in irradiance apparently caused populations to emerge occasionally before sunset or to leave the surface after the beginning of nautical twilight near dawn. Emergence before sunset was marked by slower-than-average ascent, and departure from the surface after initiation of nautical twilight was marked by faster-than-average descent. This pattern would be expected for populations avoiding visual predators by concealment in dark water. Mean ascent (0.29 ± 0.03 cm s−1 [±1 SE]) and descent (−0.26 ± 0.02 cm s−1) velocities showed little difference in magnitude, suggesting that a similar mechanism controls both. Ascent initiation times for the first such event of the night were consistent with a cue based on relative rate of change in light intensity and inconsistent with either the progress of isolumes or with absolute rate of change in light intensity.

A shallow scattering layer: High‐resolution acoustic analysis of nocturnal vertical migration from the seabed

Using bottom‐mounted, high‐frequency, inverted echo sounders (the Tracor acoustic profiling system) to examine acoustic backscattering every 2 min in 50‐cm range bins above the seabed, we recorded strong, remarkably regular, nightly migrations of organisms from ≥0.5 m above the seabed to the upper pelagic layers over a period of 6 d in July—August 1995. Population migration speeds on ascent versus descent (mean ± 1 SD) were not significantly different when estimated from the mode (or leading/trailing edge) of the population and were 0.59 ± 0.24 cm s−1 in ascent (0.96 ± 0.25 cm s−1 leading edge) and 0.67 ± 0.13 cm s−1 on descent (0.78 ± 0.14 cm s−1 trailing edge). The SD in start times of the ascent over the six nights was only 3 min, and that in the times of the completion of the descent was 7 min. Emergence began after and reentry was completed before downwelling irradiance at photosynthetically active wavelengths 1 m above bottom reached 0.5 X 10−6 µmol quanta cm−2 s−1. Migrators increased column scattering strength at 265 kHz by a factor of 14. Emergence‐trap samples and nighttime plankton tows suggested that the migrating population was dominated by mysids, mostly Neomysis kadiakensis. Our acoustic results suggest that emergence is much more prevalent and important in coastal and estuarine environments than trapping and net sampling can reveal and hence that this important component of benthic‐pelagic coupling requires additional attention both in measurements and in theoretical treatments.

Multifrequency acoustical volume backscattering patterns in the Arabian Sea--265 kHz to 3 MHz

High-frequency acoustical volume backscattering was examined during three periods of the annual cycle of winds in the Arabian Sea-the Winter Northeast (NE) Monsoon, the Summer Southwest (SW) Monsoon, and the Fall Intermonsoon-using a multifrequency acoustic profiling system deployed on an undulating towed body. This system measured acoustical volume backscattering strengths at six frequencies ranging from 265 kHz to 3.0 MHz. Measurements were made from near the surface to 250 m depth along a ship track that paralleled the coast of Oman, then proceeded away from the coast towards the middle of the Arabian Sea. At all frequencies, volume backscattering strength was highest near the surface, decreasing with increasing depth. Contrary to expectation, backscattering was generally lower during the Summer SW Monsoon than during the Winter NE Monsoon. Also contrary to expectation, backscattering was not appreciably higher near the coast than it was offshore during the Summer SW Monsoon, although it was higher near the coast during the Winter NE Monsoon. Generally speaking, regional and seasonal differences were smaller than expected, and much smaller than the fine-scale spatial and temporal variability, particularly at frequencies below 1 MHz. There was a daily pattern of increased nighttime backscattering in the upper 100 m at 265 and 420 kHz. This pattern was less evident at 1.1 MHz and above.

Variation in the Vertical Distribution of Zooplankton and Fine Particles in an Estuarine Inlet of South Carolina

Patterns in the vertical distributions of estuarine plankton within the tidally dominated North Edisto Inlet, South Carolina, were described from acoustic backscattering samples at four high frequencies (265 kHz, 420 kHz, 1.1 MHz, 3.0 MHz). Samples were collected between the near-bottom (1 m) and near-surface (within 2 m) at 0.5-m intervals every 2 min for 21 d in June and 24 d in September–October 1994. Spring and neap tidal conditions were illustrated during both seasons to represent the limits of patterns observed in the 45 d of acoustic data. Inverse calculations interrelating the returns from the four frequencies allowed estimates of biovolume density (mm3 m−3) by size class for backscatterers at selected equivalent spherical radii (ESR). Distributions of very small scatterers (0.13 mm ESR), probably representing a composite of inorganic and organic fine materials, detritus, benthic meiofauna, phytoplankton, and very small zooplankton, indicated cyclic resuspension in proportion to tidal current velocities. Mid-sized scatteres (0.79 mm ESR), probably representing much of the zooplankton population (i.e., shrimp postlarvae and crab megalopae), formed subsurface layers during the daytime. These layers fluctuated in depth temporally and were directly above the areas occupied by the small scatterers, in an apparent avoidance of high concentrations of the fine materials. Zooplankton was widely distributed vertically during nighttime in June. Large scatterers (5.0 mm ESR), probably fish larvae or small pelagic fish, occurred in vertical patterns similar to those of the zooplankton, but their distributions often were slightly higher in the water column than those of the zooplankton. The highest vertical distributions of each size class occurred in June, corresponding to seasonal differences in current velocities. The simultaneous application of multi-frequency acoustical techniques with complementary hydrographic data allows detailed descriptions of the distributional interrelationships between plankton scatterers of different sizes and some environmental parameters. This information should contribute much to interpretations of catches of esturine plankton from net samples collected at specific times, depths, and tidal stages.