Coupling Gold Nanospheres into Nanochain Constructs for High-Contrast, Longitudinal Photoacoustic Imaging.

Structural parameters play a crucial role in determining the electromagnetic and thermal responses of gold nanoconstructs (GNCs) at near-infrared (NIR) wavelengths. Therefore, developing GNCs for reliable, high-contrast photoacoustic imaging has been focused on adjusting structural parameters to achieve robust NIR light absorption with photostability. In this study, we introduce an efficient photoacoustic imaging contrast agent: gold sphere chains (GSCs) consisting of plasmonically coupled gold nanospheres. The chain geometry results in enhanced photoacoustic signal generation originating from outstanding photothermal characteristics compared to traditional gold contrast agents, such as gold nanorods. Furthermore, the GSCs produce consistent photoacoustic signals at laser fluences within the limits set by the American National Standards Institute. The exceptional photoacoustic response of GSCs allows for high-contrast photoacoustic imaging over multiple imaging sessions. Finally, we demonstrate the utility of our GSCs for molecular photoacoustic cancer imaging, both in vitro and in vivo, through the integration of a tumor-targeting moiety.

Magnetically Steered Cell Therapy For Functional Restoration Of Intraocular Pressure Control In Open-Angle Glaucoma.

Trabecular meshwork (TM) cell therapy has been proposed as a next-generation treatment for elevated intraocular pressure (IOP) in glaucoma, the most common cause of irreversible blindness. Using a magnetic cell steering technique with excellent efficiency and tissue-specific targeting, we delivered two types of cells into a mouse model of glaucoma: either human adipose-derived mesenchymal stem cells (hAMSCs) or induced pluripotent cell derivatives (iPSC-TM cells). We observed a 4.5 [3.1, 6.0] mmHg or 27% reduction in intraocular pressure (IOP) for nine months after a single dose of only 1500 magnetically-steered hAMSCs, associated with restoration of function to the conventional outflow pathway, as judged by increased outflow facility and TM cellularity. iPSC-TM cells were also effective, but less so, showing only a 1.9 [0.4, 3.3] mmHg or 13% IOP reduction and increased risk of tumorigenicity. In both cases, injected cells remained detectable in the iridocorneal angle three weeks post-transplantation. Based on the locations of the delivered cells, the mechanism of IOP lowering is most likely paracrine signaling. We conclude that magnetically-steered hAMSC cell therapy has potential for long-term treatment of ocular hypertension in glaucoma. One Sentence Summary: A novel magnetic cell therapy provided effective intraocular pressure control in a mouse model of glaucoma, motivating future translational studies.

Applications of the Photocatalytic and Photoacoustic Properties of Gold Nanorods in Contrast-Enhanced Ultrasound and Photoacoustic Imaging.

The applications of ultrasound imaging are often limited due to low contrast, which arises from the comparable acoustic impedance of normal tissues and disease sites. To improve the low contrast, we propose a contrast agent called gas-generating laser-activatable nanorods for contrast enhancement (GLANCE), which enhances ultrasound imaging contrast in two ways. First, GLANCE absorbs near-infrared lasers and generates nitrogen gas bubbles through the photocatalytic function of gold nanorods and photolysis of azide compounds. These gas bubbles decrease the acoustic impedance and highlight the injection site from the surrounding tissues. Second, GLANCE exhibits photoacoustic properties owing to the gold nanorods that emit photoacoustic signals upon laser irradiation. Additionally, GLANCE offers several benefits for biomedical applications such as nanometer-scale size, adjustable optical absorption, and biocompatibility. These distinctive features of GLANCE would overcome the limitations of conventional ultrasound imaging and facilitate the accurate diagnosis of various diseases.

Hyper-Branched Gold Nanoconstructs for Photoacoustic Imaging in the Near-Infrared Optical Window.

In plasmonic nanoconstructs (NCs), fine-tuning interparticle interactions at the subnanoscale offer enhanced electromagnetic and thermal responses in the near-infrared (NIR) wavelength range. Due to tunable electromagnetic and thermal characteristics, NCs can be excellent photoacoustic (PA) imaging contrast agents. However, engineering plasmonic NCs that maximize light absorption efficiency across multiple polarization directions, i.e., exhibiting blackbody absorption behavior, remains challenging. Herein, we present the synthesis, computational simulation, and characterization of hyper-branched gold nanoconstructs (HBGNCs) as a highly efficient PA contrast agent. HBGNCs exhibit remarkable optical properties, including strong NIR absorption, high absorption efficiency across various polarization angles, and superior photostability compared to conventional standard plasmonic NC-based contrast agents such as gold nanorods and gold nanostars. In vitro and in vivo experiments confirm the suitability of HBGNCs for cancer imaging, showcasing their potential as reliable PA contrast agents and addressing the need for enhanced imaging contrast and stability in bioimaging applications.

In Vivo Photoacoustic Monitoring of Stem Cell Location and Apoptosis with Caspase-3-Responsive Nanosensors.

Stem cell therapy has immense potential in a variety of regenerative medicine applications. However, clinical stem cell therapy is severely limited by challenges in assessing the location and functional status of implanted cells in vivo. Thus, there is a great need for longitudinal, noninvasive stem cell monitoring. Here we introduce a multidisciplinary approach combining nanosensor-augmented stem cell labeling with ultrasound guided photoacoustic (US/PA) imaging for the spatial tracking and functional assessment of transplanted stem cell fate. Specifically, our nanosensor incorporates a peptide sequence that is selectively cleaved by caspase-3, the primary effector enzyme in mammalian cell apoptosis; this cleavage event causes labeled cells to show enhanced optical absorption in the first near-infrared (NIR) window. Optimization of labeling protocols and spectral characterization of the nanosensor in vitro showed a 2.4-fold increase in PA signal from labeled cells during apoptosis while simultaneously permitting cell localization. We then successfully tracked the location and apoptotic status of mesenchymal stem cells in a mouse hindlimb ischemia model for 2 weeks in vivo, demonstrating a 4.8-fold increase in PA signal and spectral slope changes in the first NIR window under proapoptotic (ischemic) conditions. We conclude that our nanosensor allows longitudinal, noninvasive, and nonionizing monitoring of stem cell location and apoptosis, which is a significant improvement over current end-point monitoring methods such as biopsies and histological staining of excised tissue.

Photoacoustic-guided ultrasound thermal imaging without prior knowledge of tissue composition.

Thermal strain imaging (TSI) is a widely investigated ultrasound (US) thermometry technique that is based on the temperature-dependent change in speed of sound. However, a major challenge of TSI is a calibration process to account for material-dependent thermal strain. In this study, we leverage nanoparticle (NP)-mediated photoacoustic (PA) thermometry to calibrate thermal strain and guide US thermal imaging. By controlling the molecular composition of the sub-micrometer layer surrounding the NPs, PA thermometry becomes independent of the thermal characteristics of the overall background tissue where the NPs reside. Thus accurate temperature measurements are obtainable from sparse NP-mediated PA signals. These measurements are used to guide TSI, allowing US thermometry to produce an expanded temperature map over the entire region of interest without prior knowledge of tissue composition. Our feasibility study in tissue-mimicking phantoms demonstrates the potential to improve TSI by integrating a PA-based calibration method that complements and guides US thermometry.

Fluorescence-informed photoacoustic discrimination of multiple chromophores by lifetime mapping optically gated responses.

Synchronously Amplified Photoacoustic Image Recovery (SAPhIRe) offers improved background suppression using non-linear properties of modulatable contrast agents. Using SAPhIRe, multiple contrast agents in the same absorption window can be detected independently based on their unique triplet-state lifetimes. Here, we have demonstrated the unmixing of rose bengal and eosin Y signals from solution based on triplet-state lifetime mapping using both fluorescence and photoacoustics. Varying the pump-probe delay enables resolution and recovery of fast-decaying rose bengal and of slowly decaying eosin Y modulated photoacoustic signals, resulting from optically gated triplet state residence. Distinct images were reconstructed within tissue-mimicking phantom using the fitting coefficients of triplet-state lifetimes. Fluorescence was used to screen for modulation prior to photoacoustic imaging. The results suggest that lifetime unmixing can be utilized to simultaneously detect multiple pathologies with overlapping spectra using photoacoustic imaging.

Factors Influencing the Repeated Transient Optical Droplet Vaporization Threshold and Lifetimes of Phase Change, Perfluorocarbon Nanodroplets.

Perfluorocarbon nanodroplets (PFCnDs) are sub-micrometer emulsions composed of a surfactant-encased perfluorocarbon (PFC) liquid and can be formulated to transiently vaporize through optical stimulation. However, the factors governing repeated optical droplet vaporization (ODV) have not been investigated. In this study, we employ high-frame-rate ultrasound (US) to characterize the ODV thresholds of various formulations and imaging parameters and identify those that exhibit low vaporization thresholds and repeatable vaporization. We observe a phenomenon termed "preconditioning", where initial laser pulses generate reduced US contrast that appears linked with an increase in nanodroplet size. Variation in laser pulse repetition frequency is found not to change the vaporization threshold, suggesting that "preconditioning" is not related to residual heat. Surfactants (bovine serum albumin, lipids, and zonyl) impact the vaporization threshold and imaging lifetime, with lipid shells demonstrating the best performance with relatively low thresholds (21.6 ± 3.7 mJ/cm2) and long lifetimes (t1/2 = 104 ± 21.5 pulses at 75 mJ/cm2). Physiological stiffness does not affect the ODV threshold and may enhance nanodroplet stability. Furthermore, PFC critical temperatures are found to correlate with vaporization thresholds. These observations enhance our understanding of ODV behavior and pave the way for improved nanodroplet performance in biomedical applications.

In vivo shear wave elasticity imaging for assessment of diaphragm function in muscular dystrophy.

Duchenne muscular dystrophy (DMD) causes patients to suffer from ambulatory disability and cardiorespiratory failure, the latter of which leads to premature death. Due to its role in respiration, the diaphragm is an important muscle for study. A common method for evaluating diaphragm function is ex vivo force testing, which only allows for an end point measurement. In contrast, ultrasound shear wave elastography imaging (US-SWEI) can assess diaphragm function over time; however, US-SWEI studies in dystrophic patients to date have focused on the limbs without preclinical studies. In this work, we used US-SWEI to estimate the shear wave speed (SWS) in diaphragm muscles of healthy (WT) mice, mdx mice, and mdx mice haploinsufficient for utrophin (mdx-utr) at 6 and 12 months of age. Diaphragms were then subjected to ex vivo force testing and histological analysis at 12 months of age. Between 6 and 12 months, a 23.8% increase in SWS was observed in WT mice and a 27.8% increase in mdx mice, although no significant difference was found in mdx-utr mice. Specific force generated by mdx-utr diaphragms was lower than that of WT diaphragms following twitch stimulus. A strong correlation between SWS and collagen deposition was observed, as well as between SWS and muscle fiber size. Together, these data demonstrate the ability of US-SWEI to evaluate dystrophic diaphragm functionality over time and predict the biochemical and morphological make-up of the diaphragm. Additionally, our results highlight the advantage of US-SWEI over ex vivo testing by obtaining longitudinal measurements in the same subject. STATEMENT OF SIGNIFICANCE: In DMD patients, muscles experience cycles of regeneration and degeneration that contribute to chronic inflammation and muscle weakness. This pathology only worsens with time and leads to muscle wasting, including in respiratory and cardiac muscles. Because respiratory failure is a major contributor to premature death in DMD patients, the diaphragm muscle is an important muscle to evaluate and treat over time. Currently, diaphragm function is assessed using ex vivo force testing, a technique that only allows measurement at sacrifice. In contrast, ultrasonography, particularly shear wave elasticity imaging (USSWEI), is a promising tool for longitudinal assessment; however, most US-SWEI in DMD patients aimed for limb muscles only with the absence of preclinical studies. This work broadens the applications of US-SWE imaging by demonstrating its ability to track properties and function of dystrophic diaphragm muscles longitudinally in multiple dystrophic mouse models.

Improved magnetic delivery of cells to the trabecular meshwork in mice.

Glaucoma is the leading cause of irreversible blindness worldwide and its most prevalent subtype is primary open angle glaucoma (POAG). One pathological change in POAG is loss of cells in the trabecular meshwork (TM), which is thought to contribute to ocular hypertension and has thus motivated development of cell-based therapies to refunctionalize the TM. TM cell therapy has shown promise in intraocular pressure (IOP) control, but existing cell delivery techniques suffer from poor delivery efficiency. We employed a novel magnetic delivery technique to reduce the unwanted side effects of off-target cell delivery. Mesenchymal stem cells (MSCs) were labeled with superparamagnetic iron oxide nanoparticles (SPIONs) and after intracameral injection were magnetically steered towards the TM using a focused magnetic apparatus ("point magnet"). This technique delivered the cells significantly closer to the TM at higher quantities and with more circumferential uniformity compared to either unlabeled cells or those delivered using a "ring magnet" technique. We conclude that our point magnet cell delivery technique can improve the efficiency of TM cell therapy and in doing so, potentially increase the therapeutic benefits and lower the risk of complications such as tumorigenicity and immunogenicity.

pH-responsive ratiometric photoacoustic imaging of polyaniline nanoparticle-coated needle for targeted cancer biopsy.

Cancer microenvironment exhibits lower pH compared to healthy tissues, a characteristic which can be exploited using a pH-responsive needle to increase the accuracy of cancer biopsy. A needle, coated with pH-responsive polyaniline (PANI) nanoparticles (PANI-needle), is developed for the minimally invasive and quantitative pH analysis of tissue based on ratiometric photoacoustic (PA) imaging. The ratiometric PA signal from the PANI-needle within the 850-700 nm wavelength range shows a linear response as pH changes from 7.5 to 6.5. Owing to the high surface area of nanostructured PANI, the PA signal of PANI-needle exhibits a fast and reversible response of less than a few seconds. In a tissue-mimicking hydrogel phantom composed of two regions with different pH, PA ratios of PANI-needle successfully differentiate the local pH. The PANI-needle coupled with ultrasound-guided PA imaging is a promising technology for detection of malignant tissue through quantitative pH analysis during needle biopsy.

Real-time monitoring of NIR-triggered drug release from phase-changeable nanodroplets by photoacoustic/ultrasound imaging.

Optical-responsive nanodroplets have recently been studied as a new mode of remotely controlled drug delivery. As a class of new emerging smart drug carriers, NIR-absorber-loaded perfluorocarbon nanodroplets can be converted into gas bubbles through laser stimulation, called optical droplet vaporization (ODV), which provides a potential strategy to deliver therapeutic agents to solid tumors on demand. However, there is a lack of suitable technologies to monitor these drug-loaded nanodroplet behaviors in vivo, and control the site and amount of drug released. In this study, ultrasound and photoacoustic imaging technology were applied to directly monitor optical-responsive, drug-loaded nanodroplets within the tissue. We explored the effects of laser energy, repetition rate, and number of pulses on the release profiles of the delivered drug as well as ultrasound and photoacoustic imaging signal-intensity curves. The conducted studies demonstrated that this noninvasive technology helped determine the optimum time point for laser activation on accumulated drug-loaded nanodroplets within tissues, allowing for the potential to effectively treat pathologies while minimizing drug-related toxicities.

Characterizing a photoacoustic and fluorescence imaging platform for preclinical murine longitudinal studies.

Significance: To effectively study preclinical animal models, medical imaging technology must be developed with a high enough resolution and sensitivity to perform anatomical, functional, and molecular assessments. Photoacoustic (PA) tomography provides high resolution and specificity, and fluorescence (FL) molecular tomography provides high sensitivity; the combination of these imaging modes will enable a wide range of research applications to be studied in small animals. Aim: We introduce and characterize a dual-modality PA and FL imaging platform using in vivo and phantom experiments. Approach: The imaging platform's detection limits were characterized through phantom studies that determined the PA spatial resolution, PA sensitivity, optical spatial resolution, and FL sensitivity. Results: The system characterization yielded a PA spatial resolution of 173 ± 17 µ m in the transverse plane and 640 ± 120 µ m in the longitudinal axis, a PA sensitivity detection limit not less than that of a sample with absorption coefficient µ a = 0.258 cm - 1 , an optical spatial resolution of 70 µ m in the vertical axis and 112 µ m in the horizontal axis, and a FL sensitivity detection limit not < 0.9 µ M concentration of IR-800. The scanned animals displayed in three-dimensional renders showed high-resolution anatomical detail of organs. Conclusions: The combined PA and FL imaging system has been characterized and has demonstrated its ability to image mice in vivo, proving its suitability for biomedical imaging research applications.

A Scholte wave approach for ultrasonic surface acoustic wave elastography.

BACKGROUND: Pathological changes in tissues are often related to changes in tissue mechanical properties, making elastography an important tool for medical applications. Among the existing elastography methods, ultrasound elastography is of great interest due to the inherent advantages of ultrasound imaging technology, such as low cost, portability, safety, and wide availability. Although ultrasonic shear wave elastography, as a platform technology, can potentially quantify the elasticity of tissue at any depth, its current implementation cannot assess superficial tissue but can only image deep tissue. PURPOSE: To address this challenge, we proposed an ultrasonic Scholte-wave-based approach for imaging the elasticity of superficial tissue. METHODS: The feasibility of the proposed technique was tested using a gelatin phantom with a cylindrical inclusion. To generate Scholte wave in the superficial region of the phantom, we proposed a new experimental configuration in which a liquid layer was introduced between an ultrasound imaging transducer and the tissue-mimicking phantom. We utilized an acoustic radiation force impulse to excite the tissue-mimicking phantom, analyzed the properties of the generated Scholte waves, and applied the waves for elasticity imaging. RESULTS: In this study, we first reported the observation that Scholte (surface) waves and shear (bulk) waves were simultaneously generated, and they propagated in the superficial and deeper regions of the phantom, respectively. Then, we presented some important properties of the generated Scholte waves. For a 5w/v% gelatin phantom, the generated Scholte waves have a speed of around 0.9 m/s, a frequency of about 186 Hz, and thus a wavelength of about 4.8 mm. The speed ratio between the simultaneously generated Scholte wave and shear wave is about 0.717, which is 15% lower than the theoretical expectation. And we further demonstrated the feasibility of Scholte wave as a mechanism for imaging superficial tissue elasticity. Together with the simultaneously generated shear wave, the Scholte wave was shown to be able to quantitatively image both the background and the cylindrical inclusion (4 mm in diameter) of the tissue-mimicking gelatin phantom. CONCLUSIONS: This work shows that the elasticity of superficial tissue can be evaluated by utilizing the generated Scholte wave alone, and it also shows that a comprehensive elasticity imaging of the tissue extending from the superficial to deep regions can be achieved by combining the proposed Scholte wave technique and the conventional shear wave technique.

Tunable Interparticle Connectivity in Gold Nanosphere Assemblies for Efficient Photoacoustic Conversion.

Manipulating matter at the nanometer scale to create desired plasmonic nanostructures holds great promise in the field of biomedical photoacoustic (PA) imaging. We demonstrate a strategy for regulating PA signal generation from anisotropic nano-sized assemblies of gold nanospheres (Au NSs) by adjusting the inter-particle connectivity between neighboring Au NSs. The inter-particle connectivity is controlled by modulating the diameter and inter-particle spacing of Au NSs in the nanoassemblies. The results indicate that nanoassemblies with semi-connectivity, i.e., assemblies with a finite inter-particle spacing shorter than the theoretical limit of repulsion between nearby Au NSs, exhibit 3.4-fold and 2.4-fold higher PA signals compared to nanoassemblies with no connectivity and full connectivity, respectively. Furthermore, due to the reduced diffusion of Au atoms, the semi-connectivity Au nanoassemblies demonstrate high photodamage threshold and, therefore, excellent photostability at fluences above the current American National Standards Institute limits. The exceptional photostability of the semi-connectivity nanoassemblies highlights their potential to surpass conventional plasmonic contrast agents for continuing PA imaging. Collectively, our findings indicate that semi-connected nanostructures are a promising option for reliable, high-contrast PA imaging applications over multiple imaging sessions due to their strong PA signals and enhanced photostability.

Noninvasive ultrasound assessment of tissue internal pressure using dual mode elasticity imaging: a phantom study.

Objective.Tissue internal pressure, such as interstitial fluid pressure in solid tumors and intramuscular pressure in compartment syndrome, is closely related to the pathological state of tissues. It is of great diagnostic value to measure and/or monitor the internal pressure of targeted tissues. Because most of the current methods for measuring tissue pressure are invasive, noninvasive methods are highly desired. In this study, we developed a noninvasive method for qualitative assessment of tissue internal pressure based on a combination of two ultrasound elasticity imaging methods: strain imaging and shear wave elasticity imaging.Approach.The method was verified through experimental investigation using two tissue-mimicking phantoms each having an inclusion confined by a membrane, in which hydrostatic pressures can be applied and maintained. To examine the sensitivity of the elasticity imaging methods to pressure variation, strain ratio and shear modulus ratio (SMR) between the inclusion and background of phantom were obtained.Main results.The results first experimentally prove that pressure, in addition to elasticity, is a contrast mechanism of strain imaging, and further demonstrate that a comparative analysis of strain ratio and SMR is an effective method for noninvasive tissue internal pressure detection.Significance.This work provides a new perspective in interpreting the strain ratio data in medical diagnosis, and it also provides a noninvasive alternative for assessing tissue internal pressure, which could be valuable for the diagnosis of pressure-related diseases.

Correction: In vivo photoacoustic image-guided tumor photothermal therapy and real-time temperature monitoring using a core-shell polypyrrole@CuS nanohybrid.

Correction for 'In vivo photoacoustic image-guided tumor photothermal therapy and real-time temperature monitoring using a core-shell polypyrrole@CuS nanohybrid' by Yang Cao et al., Nanoscale, 2022, 14, 12069-12076, https://doi.org/10.1039/D2NR02848D.

High-Frequency Ultrasound Imaging With Sub-Nyquist Sampling.

Implementation of a high-frequency ultrasound (HFUS) beamformer is computationally challenging because of its high sampling rate. This article introduces an efficient beamformer with sub-Nyquist sampling (or bandpass sampling) that is suitable for HFUS imaging. Our approach used channel radio frequency data sampled at bandpass sampling rate (i.e., 4/ 3fc ) and postfiltering-based interpolation to reduce the computational complexity. A polyphase structure for interpolation was used to further reduce the computational burden while maintaining an adequate delay resolution ( δ ). The performance of the proposed beamformer (i.e., 4/ 3fc sampling with sixfold interpolation, δ = 8fc ) was compared with that of the conventional method (i.e., 4fc sampling with fourfold interpolation, δ = 16fc ). Ultrafast coherent compounding imaging was used in simulation, in vitro and in vivo imaging experiments. Axial/lateral resolution and contrast-to-noise ratio (CNR) values were measured for quantitative evaluation. The number of transmit pulse cycles was varied from 1 to 3 using two transducers with different fractional bandwidths (67% and 98%). In the simulation, the proposed and conventional methods showed the similar -6-dB axial beam widths (63.5 and 61.5 µm , respectively) from the two-cycle transmit pulse using the transducer with a bandwidth of 67%. In vitro and in vivo imaging experiments were performed using a Verasonics ultrasound research platform equipped with a high-frequency array transducer (20-46 MHz). The in vitro imaging results using a wire target showed consistent results with the simulation study (i.e., disparity at -6-dB axial resolution). The in vivo feasibility study with a murine mouse model with breast cancer was also performed, and the proposed method yielded a similar image quality compared with the conventional method. From these studies, it was demonstrated that the proposed HFUS beamformer based on the bandpass sampling can substantially reduce the computational complexity while minimizing the loss of spatial resolution for HFUS imaging.

In vivo safety study using radiation at wavelengths and dosages relevant to intravascular imaging.

SIGNIFICANCE: Intravascular photoacoustic (IVPA) imaging can identify native lipid in atherosclerotic plaques in vivo. However, the large number of laser pulses required to produce 3D images is a safety concern that has not been fully addressed. AIM: We aim to evaluate if irradiation at wavelengths and dosages relevant to IVPA imaging causes target vessel damage. APPROACH: We irradiate the carotid artery of swine at one of several energy dosages using radiation at 1064 or 1720 nm and use histological evaluation by a pathologist to identify dose-dependent damage. RESULTS: Media necrosis was the only dose-dependent form of injury. Damage was present at a cumulative fluence of 50 J / cm2 when using 1720 nm light. Damage was more equivocally identified at 700 J / cm2 using 1064 nm. CONCLUSIONS: In prior work, IVPA imaging of native lipid in swine has been successfully conducted below the damage thresholds identified. This indicates that it will be possible to use IVPA imaging in a clinical setting without damaging vessel tissue. Future work should determine if irradiation causes an increase in blood thrombogenicity and confirm whether damaged tissue will heal over longer time points.

Gold nanoparticles conjugated with DNA aptamer for photoacoustic detection of human matrix metalloproteinase-9.

Matrix metalloproteinase-9 (MMP-9) plays major roles in extracellular matrix (ECM) remodeling and membrane protein cleavage, suggesting a high correlation with cancer cell invasion and tumor metastasis. Here, we present a contrast agent based on a DNA aptamer that can selectively target human MMP-9 in the tumor microenvironment (TME) with high affinity and sensitivity. Surface modification of plasmonic gold nanospheres with the MMP-9 aptamer and its complementary sequences allows the nanospheres to aggregate in the presence of human MMP-9 through DNA displacement and hybridization. Aggregation of gold nanospheres enhances the optical absorption in the first near-infrared window (NIR-I) due to the plasmon coupling effect, thereby allowing us to detect the aggregated gold nanospheres within the TME via ultrasound-guided photoacoustic (US/PA) imaging. Selective and sensitive detection of human MMP-9 via US/PA imaging is demonstrated in solution of nanosensors with the pre-treatment of human MMP-9, in vitro in cell culture, and in vivo in a xenograft murine model of human breast cancer.