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Logo of nihpaAbout Author manuscriptsSubmit a manuscriptNIH Public Access; Author Manuscript; Accepted for publication in peer reviewed journal;
Mol Pharm. Author manuscript; available in PMC Mar 4, 2014.
Published in final edited form as:
PMCID: PMC3593826
NIHMSID: NIHMS442165

Multifunctional gold nanoparticles for diagnosis and therapy of disease

Abstract

Gold nanoparticles (AuNPs) have a number of physical properties that make them appealing for medical applications. For example, the attenuation of X-rays by gold nanoparticles has led to their use in computed tomography imaging and as adjuvants for radiotherapy. AuNPs have numerous other applications in imaging, therapy and diagnostic systems. The advanced state of synthetic chemistry of gold nanoparticles offers precise control over physicochemical and optical properties. Furthermore gold cores are inert and are considered to be biocompatible and non-toxic. The surface of gold nanoparticles can easily be modified for a specific application and ligands for targeting, drugs or biocompatible coatings can be introduced. AuNPs can be incorporated into larger structures such as polymeric nanoparticles or liposomes that deliver large payloads for enhanced diagnostic applications, efficiently encapsulate drugs for concurrent therapy or add additional imaging labels. This array of features has led to the afore-mentioned applications in biomedical fields, but more recently in approaches where multifunctional gold nanoparticles are used for multiple methods, such as concurrent diagnosis and therapy, so called theranostics. The following review covers basic principles and recent findings in gold nanoparticle applications for imaging, therapy and diagnostics, with a focus on reports of multifunctional AuNPs.

Keywords: nanoparticles, multimodality, imaging, gold, theranostics, diagnostics

INTRODUCTION

Nanoscale structures can exhibit widely different properties to bulk materials or small molecules, which renders them applicable in the fields of medical imaging and therapy.14 For example, the absorbance and fluorescence of gold nanoparticles (AuNPs) is much greater compared with bulk gold, and can be tuned from the visible to the near infrared (NIR) region by changing nanostructure size and morphology.57 Tissue absorbs light weakly in the NIR window, making this window ideal for optically based applications. These appealing properties have resulted in an outburst of efforts to explore synthetic routes to produce gold nanostructures of different shapes, examples of which are shown in Figure 1A. Another highly useful feature is the electromagnetic field enhancement by sharp and spiky edges of the nanostructures, such as stars or nanorods, which can be used in surface enhanced Raman spectroscopy imaging.811 The optical scattering of AuNPs can be harnessed to detect them with a variety of microscopy methods.12 AuNPs absorb X-rays strongly and can thus be used as contrast agents for X-ray based imaging techniques and as adjuvants for radiotherapy.13, 14 Moreover, AuNPs can transform absorbed light into heat, resulting in localized temperature rises,6, 1517 which can be used to provide contrast for photoacoustic imaging1820 or for photothermal therapy.17, 2134

Figure 1
Examples of different gold nanostructures (A) and examples of engineering of biocompatible gold nanoparticles through coating or encapsulation into carriers (B). (C) Aschematic depiction of gold nanoparticles for in vivo use. Figure adapted, with permission, ...

Further therapeutic applications are as drug or gene delivery vehicles and, in the case of AuNPs formed from Au-198, as radiotherapeutics.3547 Due to these properties, AuNPs are now widely explored for their potential as diagnostic or therapeutic agents in a variety of medical fields. Furthermore, the successes of these agents has led to FDA-approvals for AuNP-based in vitro diagnostic systems and clinical trials of AuNPs as cancer and cardiovascular treatments.4853

Nanoparticle multifunctionality is accomplished by combining different properties for imaging, targeting, or therapeutic delivery into one single platform. Such multifunctional platforms can serve as imaging reporters that provide complementary information or as theranostic agents, i.e. probes with concurrent imaging and therapeutic features. In the case of AuNPs, multifunctional approaches can simultaneously exploit multiple properties of the gold core, such as contrast for computed tomography (CT) imaging and the photothermal effect for therapy.54 Alternatively, one of the effects could come from another substance incorporated into the same platform as gold cores. An example, is the use of AuNPs to deliver drugs and optical imaging of the gold cores to track delivery.55 In this review we will briefly discuss AuNP synthesis, before surveying their abovementioned imaging, therapeutic and in vitro diagnostic applications. We will emphasize examples where multifunctional AuNPs have been used and highlight FDA-approved applications and clinical trials of AuNPs.

AUNP SYNTHESIS AND STRUCTURE

The most widely applied and simplest methods to produce AuNPs use chemical reduction of gold salt to metallic gold in the presence of a capping ligand, with the Turkevich and Brust-Schiffrin methods being the standards for aqueous and organic-based synthesis, respectively.56, 57 In the Turkevich method, gold chloride is dissolved in water, heated to boiling point and sodium citrate is added, causing the gold salt to be reduced and gold cores to be formed.57 The reaction results in water-soluble, citrate capped AuNPs whose size can be tuned from 15–150 nm.58 They are not stable in saline solutions, and require coating substitution for biological applications (vide infra).50 In the Brust-Schiffrin method, gold chloride is transferred from water to toluene via use of a phase transfer chemical.56 A thiol such as dodecanethiol is added to act as a capping ligand. Sodium borohydride in water is added to reduce the gold. The AuNPs produced are highly stable, range in size from 1–5 nm and are soluble in non-polar solvents.59 Use in biological media requires, therefore, ligand exchange46 or additional coating with amphiphilic polymers or lipids.60

Other synthetic approaches include physical approaches such as microwaves and UV irradiation to nucleate the AuNPs or biological routes, so called green syntheses, with plant extracts or microorganism assisted formation of the nanostructures.61, 62 Variations of these methods include seeded growth, where previously synthesized AuNPs are used as seeds to nucleate further growth of gold into other shapes. This is usually accomplished in the presence of surfactants and mild reducing agents or with templates.63 Seeded growth is readily used to produce multi-shaped gold nanostructures such as rods, triangles,64 cubes,65 platelets or stars.66

Gold nanostructures are synthesized with different capping ligands that provide their solubility in aqueous or organic solvents, grant stability, and prevent aggregation. Although gold nanostructures have a rather inert and unreactive noble metal core, their biological applications often require further surface functionalization to assure high biocompatibility and low cytotoxicity.67 Gold surfaces react easily with thiol (SH) groups forming stable Au:S bonds.68 Therefore, thiolated ligands can be used to introduce targeting moieties, different functional groups for further reactions, or drug molecules, among other options.69 One of the most important requirements for biological applications is a modification of the nanostructure surface with a biocompatible coating to reduce uptake by the reticuloendothelial system and prevent nonspecific binding to biological substances.70

A variety of coating methods have been proposed to increase the biocompatibility of AuNPs, such as ligand substitution, amphiphile coating or embedding in a carrier matrix.67 The most widely applied coating polymer is polyethylene glycol (PEG), which is neutral in charge and highly hydrophilic, thereby preventing nonspecific protein adsorption on the nanoparticle surface, uptake by the reticuloendothelial system and providing lengthened blood circulation times.7173 Other examples include polyelectrolyte layer-by-layer wrapping, coating with proteins such as bovine serum albumin74 or silica75 coatings. AuNPs can also be incorporated into larger particles made of polymers or lipids, such as liposomes,76 micelles77, PLGA nanoparticles78 or dendrimers Figure 1B.79, 80 This approach facilitates the integration of multiple components, such as additional diagnostic or therapeutic materials, giving a route to engineer complex multifunctional nanoparticle platforms and broadening the applications of AuNPs. A generalized schematic of a AuNP for in vivo use is schematically depicted in Figure 1C.

AuNP targeting can be achieved in a passive fashion, where long circulating AuNPs can accumulate in cancers by penetrating the leaky tumor vasculature. This is known as the enhanced permeability and retention (EPR) effect.81 Alternatively, AuNPs can be targeted to specific cell types, receptors or proteins via attachment of targeting ligands such as antibodies, proteins, peptides, aptamers and small molecules.12, 55, 82, 83

IMAGING

X-ray based imaging

The high atomic number and electron density of Au leads to efficient absorption of X-ray irradiation, superior to conventional iodine-based contrast agents currently used in the clinic,13 especially at higher X-ray tube voltages, such as 120 and 140 kV. Additionally, AuNPs can offer longer circulation times than conventional agents, enabling prolonged imaging, targeting to specific cell types or other ligands and cell tracking.82, 84, 85 Also, the payloads of AuNPs in biocompatible carriers can be precisely controlled.78 These properties, along with the biocompatibility of gold compared with other elements that also strongly attenuate X-rays, has resulted in the exploration of AuNPs as contrast agents for X-ray based imaging techniques such as computed tomography (CT).79, 80, 8699

For example, Cai et al studied the in vivo CT contrast performance of 38 nm PEG-coated AuNP as a blood-pool contrast agent for X-ray computed tomography in mice.85 AuNPs were injected at a 493 mg Au/kg dose into mice, which were then scanned with a microCT. Long-lasting enhancements in contrast were observed in the blood vessels of the mice, with a 100 HU increase in attenuation still apparent at 24 hours post-injection. In comparison, the contrast arising from similar doses of conventional X-ray contrast agents abates within a couple of minutes. Kim et al. investigated AuNPs conjugated to a prostate-specific membrane antigen (PSMA) aptamer for specific targeting of PSMA on prostate cancer cells.98 They used a clinical CT scanner to detect specific accumulation of the AuNPs in these cells and when doxorubicin was additionally incorporated to the AuNP formulation they observed cancer cell death.

The addition of other contrast generating substances to Au nanostructures can generate multimodality probes that provide contrast for CT as well as other imaging techniques such as magnetic resonance imaging (MRI) or fluorescence.100 For instance, Alric et al. developed Au nanoparticles coated with a Gd chelate for both CT and MRI (Figure 2A).101 Examples of the contrast produced by Au-Gd nanoparticles in synchrotron radiation CT (SRCT) and MRI are shown in Figure 2A. Comparison of both SRCT and MR images of a rat taken before and after injection of the Au-Gd nanoparticles showed contrast in the kidneys, bladder and urine (Figure 2B). Analysis of the SRCT images and ex vivo ICP-MS analyses revealed urinary excretion and low accumulation in the spleen, heart, brain, liver and lungs confirming the platform’s applicability as a blood pool contrast agent.

Figure 2
Gold nanoparticles functionalized with Gd for dual modality imaging (SRCT and MRI). (A) The nanoparticle design with SRCT and MR images of vials of the agent to the left and right. (B) SRCT of a rat before (t = −2 min) and after injection (t = ...

Advancements in detector technology have allowed the development of spectral CT, a technique that is currently in experimental testing.102 Spectral CT allows multicolor imaging through splitting of the X-ray beam into six components based on the energy, enabling discrimination of different materials, which is dependent on the characteristic, energy-dependent X-ray attenuation profile of the material. The technique was applied by Cormode et al. to image and distinguish Au nanoparticles accumulated in macrophages in atherosclerotic plaques, iodine in vasculature, as well as calcified material, simultaneously.103 As these nanoparticles have the fluorophore Rhodamine incorporated, in addition to electron microscopy, their localization in macrophages was detected by immunofluorescence as well.

Spectral CT imaging has also been done with gold nanoclusters targeted with antibodies via an avidin-biotin linkage to fibrin.104 This was used for specific detection of clots created in vitro. Furthermore, sentinel lymph nodes could be identified following injection of gold nanoclusters into the foot of the mouse. Bismuth and ytterbium based nanoparticles, can also be specifically detected with spectral CT and have been targeted to fibrin, for thrombus detection.105, 106

Fluorescence imaging

The fluorescence of bulk gold, first observed by Mooradian in 1969107, is very weak with a quantum yield in the order of 10−10. Strikingly, strong fluorescence with a quantum yield of up to 10−3 is observed in AuNPs such as rods or shells.108116 Such fluorescence can be readily applied in the biomedical field, especially as it can be tuned to the NIR window (650–900 nm), a region of the electromagnetic spectrum where light penetrates tissues relatively well.117119 Au nanorods (AuNRs) exhibit transverse and longitudinal plasmon resonance bands that originate from the oscillation of the surface electrons along the x- and y-axis, respectively. For a given diameter, when the aspect ratio of the rod increases, the transverse band remains unchanged while the longitudinal band shifts to the red. For example, Mohamed et al. found that AuNRs with an aspect ratio of 2 had an emission maximum of 540 nm, while increasing the aspect ratio to 5.4 led to an emission maximum of 740 nm.120 These fluorescence properties have found use in DNA biosensing.121 To that end, Li et al. have functionalized AuNRs with DNA sequences.121 When complementary sequences were added, the AuNRs aggregate and the fluorescence signal decreased. This process was reversed upon heating the AuNRs and disaggregation. Tang et al. used AuNR for labeling of mouse intestinal blood vessels to study the morphology of the vasculature using NIR confocal microscopy.122

Park et al. were able to image the 3D distribution of luminescent gold nanoshells (AuNS) in murine tumors using two-photon induced photoluminescence.123 The AuNS were coated with polyethylene glycol and accumulated in the tumor via the EPR effect. In addition to intrinsic fluorescence, AuNPs can be made fluorescent by the addition of organic dyes. For example, Cormode et al. used this strategy to create a AuNP based probe that provides contrast for three imaging modalities, as depicted in Figure 3A. The Au nanocrystal core (CT label) was coated with a phospholipid mixture that included Rhodamine for fluorescence as well as Gd for MRI contrast.82 The contrast produced by this nanoparticle for the three modalities is shown in Figure 3B–C. The protein component of high-density lipoprotein was incorporated into the phospholipid coating, which provided structural integrity as well specificity for macrophages in atherosclerotic plaques, as indicated in Figure 3D–E.

Figure 3
A) A schematic depiction of a multimodality contrast agent with Au core providing contrast for CT and additional labels for fluorescent imaging (Rhodamine) and MRI (Gd) in the phospholipid coating. Fluorescence (B), CT (C) and MRI (D) phantoms demonstrating ...

Surface enhanced Raman spectroscopy imaging

Raman spectroscopy is a technique used to study molecular vibrations, rotations and other processes. Raman spectra are highly complex, and can be used as “fingerprints” of molecules, thus giving the chemical composition of a sample. Although highly specific, Raman spectroscopy is limited by its low sensitivity, since only one photon in 108 is Raman scattered. Absorption upon AuNP or other metal surfaces enhances the intensity of the vibrational spectra of Raman active molecules by several orders of magnitude.124 This discovery has led to emergence of a new technique called surface enhanced Raman spectroscopy (SERS). This phenomenon, which is widely observed and studied using AuNPs, is thought to be due to an electromagnetic field close to the particle surface produced by the presence of a localized surface plasmon resonance (the collective oscillation of surface electrons).125128 The strongest electromagnetic field enhancement occurs at sharp nanostructure edges like AuNR tips and between aggregated colloids. Also, the surface plasmon absorption of Au nanostructures can be tuned into the NIR, thus avoiding absorption of excitation light by biological samples and limiting the interference for the SERS signal.129 Therefore Au nanostructures are attractive as labels for flow cytometry130 and as contrast agents for biological SERS imaging.131134,135137,138

Fujita’s group investigated the entry of AuNPs into living cells with SERS imaging. Macrophage cells were incubated with 50 nm Au nanoparticles and imaged using a Raman microscope. SERS signals were observed from biomolecules near the nanoparticle surfaces. Nanoparticle movement inside the cell could be followed with high spatial resolution. Fluctuations in the Raman signal over time were attributed to the adsorption/desorption of biomolecules to the nanoparticle surface.139 Similarly, Eliasson et al. demonstrated the SERS discrimination between several intracellular components in living cells using SERS.140 Single human lymphocytes were incubated with AuNPs and Rhodamine 6G as a model analyte. SERS imaging allowed for the identification of Rhodamine 6G probe within the cell as well as spectra corresponding to DNA and nucleotides.

Qian et al. reported a SERS contrast agent where a reporter molecule (malachite green) was adsorbed onto the surface of 60 nm AuNPs.141 An additional coating of PEG was applied. The contrast arising from these nanoparticles was found to be 200 times that of quantum dots, another type of nanoparticle proposed for optical contrast. The authors demonstrated that subcutaneous injections of the AuNPs into the flank of a mouse could be detected using a SERS microscope. Furthermore, targeting the Au-NPs with an antibody against anti-epidermal growth factor receptor (EGFR) resulted in enhanced binding to tumor cells that overexpressed this receptor. Targeted and non-targeted AuNPs were injected into tumor bearing mice and both SERS spectra and ICP-MS confirmed a greater accumulation at 5 hours post-injection.

Kircher et al. employed a multifunctional approach combining SERS, MRI and photoacoustics, where a AuNP-based platform was used to image brain tumors and accurately guide surgery, offering improved identification of the tumor margins.142 The nanoparticle consisted of a 60 nm gold core coated with a highly Raman active molecule trans-1,2-bis(4-pyridyl)-ethylene and a 30 nm thick silica shell. The silica surface was further modified with Gd3+ chelates. In this design the AuNPs served as a contrast agent for photoacoustic imaging and the amplifier for the SERS Raman tag, while Gd3+ provides contrast for MRI. The photoacoustic imaging gives high spatial resolution combined with highly sensitive SERS imaging led to detailed identification of the tumor outline. The in vivo detection threshold for the nanoparticles was found to be very low, in the 50 pM range. After tail-vein injection of the agent in a tumor mouse model, the MRI contrast-to-noise ratio of the tumor increased from 2.2 ± 0.3 to 14.0 ± 1.9, the photoacoustic signal increased by 75 % and the SERS signal of the agent was detected with an SNR of 11.1. Use of this agent improved the success of resections in removing all tumor tissue.

Photoacoustic imaging

In photoacoustic imaging, the subject is irradiated with light, which results in localized heating, and a small expansion of tissue, causing a sound wave. Use of short laser pulses generates sound waves in the ultrasonic frequency range.143, 144 Photoacoustic imaging is advantageous compared to some other imaging techniques, as the radiation used is non-ionizing and the use of ultrasound as the output results in higher spatial resolution compared to optical methods due to the lower scattering of ultrasound in tissue. Since different types of biological tissues have different light absorption coefficients, this imaging modality can discriminate between tissues and has a penetration depth in the centimeter range with sub-millimeter spatial resolution.19, 145, 146 Due to their aforementioned strong absorption of light, AuNP have been explored as contrast agents for photoacoustic imaging.20 In this case also, using light in the NIR window is advantageous.147151 The range of gold nanostructures whose absorbance is adjustable to the NIR region by size or morphology (e.g. cages,152154 nanospheres,155 and nanorods155), have been applied for this imaging technique.18, 143, 156158

Agarwal et al. published one of the earliest examples of gold nanoparticles used as contrast agents for photoacoustic imaging.159 In this study gels containing AuNR were implanted in the hind limb of a mouse and imaged with a photoacoustic system. Lu et al. tested PEG-coated AuNSs for photoacoustic tomography of the vasculature in mice.160 The AuNSs were ~ 50 nm in diameter with an absorption peak precisely tuned to 800 nm. Photoacoustic imaging using these nanoparticles resulted in very high spatial resolution images of the brain vasculature, with capillaries detected of diameter as little as 100 μm. In another example Wang et al. employed PEGylated AuNPs as a contrast agent to detect macrophages in atherosclerotic plaques ex vivo. Endocytosis of AuNPs by macrophages caused AuNP aggregation inside the cells and a resultant red-shift in absorption due to plasmon resonance coupling. This allowed discrimination of the AuNPs inside macrophages from free AuNPs via the use of irradiation of different wavelengths.161 Pan et al. have developed a photoacoustic contrast agent for intravascular imaging based on an almond oil emulsion encapsulated in phospholipids carrying multiple 2–4 nm AuNPs 162, 163 The surface of the nanoparticle was decorated with the biotin ligand, which allowed targeting of fibrin via the use of an avidin labeled antibody. In vitro experiments revealed marked targeting to fibrin-rich clots, as compared to non-targeted nanoparticles.162 Non-targeted nanoparticles were used as a vascular phase agent and were found to increase the photoacoustic signal amplitude of the blood vessels of rats by 60%, when injected at a 3 Au mg/kg dose.164 In a further experiment, these AuNPs, when targeted to the αvβ3-integrin, were shown to specifically accumulate in a mouse matrigel model of angiogenesis (a process where the αvβ3-integrin is overexpressed).163

The Emelianov group used combined photoacoustic and ultrasound imaging to perform in vivo tracking of mesenchymal stem cells labeled with 20 nm AuNPs.165 Noninvasive detection of stem cells in vivo is very attractive for the optimization and monitoring of stem cell-based therapies. The labeled stem cells were embedded in a PEGylated fibrin gel, which was subsequently implanted into rats. Ultrasound provided anatomical information, while multiwavelength photoacoustic imaging was used to localize AuNP labeled cells and identify other biological components. As is shown in Figure 4, photoacoustic imaging could detect labeled cells injected in the hind limb. Furthermore, the cells could be spectrally distinguished from oxygenated hemoglobin, deoxygenated hemoglobin, and skin, as depicted in Figure 4D and H. In a multimodality approach, Qu et al. combined ultrasound, photoacoustic imaging and a technique known as magneto-motive ultrasound (MMUS) into hybrid magneto-photoacoustic (MPA) imaging.166 MMUS employs a pulsed magnetic field to make magnetically labeled tissue rapidly vibrate, producing ultrasound waves. For this combined approach, a liposomal formulation of AuNRs and iron oxide nanoparticles was developed as contrast agents for both photoacoustics and MMUS. In vitro imaging confirmed that only liposomes loaded with AuNR and iron oxide gave contrast in both techniques. Furthermore, merging the data from both techniques gave improved contrast resolution. Kim et al. reported an unusual platform composed of carbon nanotubes upon which was grown a gold shell.167 These structures were also found to be effective photoacoustic contrast agents and to produce heat upon irradiation, thus have potential as therapeutics via photothermal heating.

Figure 4
In vivo photoacoustic imaging of stem cells via labeling with AuNPs. (A–D) Ultrasound, photoacoustic, ultrasound/photoacoustic overlay, and ultrasound/spectroscopic images of gel encapsulated stem cells labeled with AuNPs injected into a rat hind ...

Optical imaging

AuNPs have extraordinary light scattering properties, which are not observed in non-plasmonic nanoparticles. Changes in size and shape of AuNPs also influence scattering, providing an opportunity to tune the agent to possess optimal light scattering performance. For example, the reflection coefficient and the optical cross section both increase with size, leading to more efficient light scattering by larger rather than smaller nanoparticles.168 Additionally, compared with fluorescent probes used in optical imaging, AuNPs do not suffer from photobleaching.169 These properties have encouraged investigations of AuNPs as contrast agents for light scattering imaging.168, 170, 171,172

For example, Qian et al. used dark field microscopy to image cancer cells for two cell cycles, monitoring resonant light scattering from AuNPs.173 The strong scattering signals enabled visualization of AuNP entry to the cell nucleus (when functionalized with RGD peptides and a nuclear location sequences) or the cytoplasm (when coupled to RGD only), and their localization was traceable after cell division. The light scattering of AuNPs was used by El-Sayed et al. to discriminate cancerous cells from healthy cells.12 35 nm AuNPs were modified with an anti-EGFR antibody and incubated with a nonmalignant epithelial cell line and two malignant oral epithelial cell lines. The malignant cell lines over-express EGFR, in comparison with the non-malignant cells. Light scattering signals from AuNPs could easily be detected and 600 % higher signal was found in malignant compared with non-malignant cells when incubated with antibody modified AuNPs. Non-targeted AuNPs did not show a significant difference in uptake between the cell lines. Furthermore, non-targeted AuNPs seemed to aggregate within the cells, as indicated by a red-shift of their absorption maximum from 545 to 552 nm, allowing secondary spectroscopic evaluation of imaging results.

THERAPY

Drug delivery

AuNPs have a range of properties that make them suitable for drug delivery. The noble metal core is inert, contributing to low toxicity and good biocompatibility, which are requirements for biological applications.174 The flexibility in AuNP size and shape facilitates selection of dimensions optimal for loading therapeutics such as proteins, peptides, oligonucleotides, or small drug molecules.69, 175, 176 Additionally, high surface area and a choice of surface chemistries facilitate the loading of not only a large therapeutic cargo but also other entities such as targeting molecules, linkers, additional contrast media and so on.177180 AuNPs have been widely studied for application in anticancer therapy, due to the great need for new treatments in this area.181186 Furthermore, AuNPs can preferentially accumulate in tumors through the EPR effect.

In a simple example, Tomuleasa et al. tested three different AuNPs formulations containing either doxorubicin, cisplatin, and capecitabine, for treatment of liver cancer.187 The drug molecules were non-covalently complexed to AuNPs coated with aspartic acid. The cell lines used were hepatocellular carcinoma cells, chemotherapy resistant hepatocellular carcinoma cells and non-cancerous liver cells. Enhanced therapeutic outcomes were observed for the cancer cells treated with AuNP-drug conjugates when compared to free drugs. Additionally, the AuNP-drug complexes were effective against the chemotherapy resistant cells.

In another study, Kumar et al. combined therapy with active targeting by functionalizing AuNPs with both therapeutic and targeting peptides.83 The PMI peptide interferes with the p53 pathway and can thus induce cancer cell apoptosis. The CRGDK peptide binds to the neuropilin-1 receptor, which is overexpressed on cancer cells. Both peptides were covalently linked to 2 nm AuNPs. The internalizing and therapeutic effects were compared using two breast cancer cell lines: MDA-MB-321, which expresses the neuropilin-1 receptor and MCF-7S, which has low neuropilin-1 expression. Incubation with these AuNPs led to stronger in vitro toxicity for MDA-MB-321 cells than MCF-7S. These and other experiments proved neuropilin-1 mediated recognition and internalization.

Heo et al. reported a complex AuNP platform that included biotin as a targeting ligand, the anticancer drug paclitaxel, Rhodamine B to facilitate fluorescence detection and PEG for enhanced biocompatibility.55 Cyclodextrin, a well known drug-host molecule, was attached to the AuNP surface for non-covalent paclitaxel inclusion. A schematic depiction of the AuNP platform is shown in Figure 5A. Biotin was used as a targeting ligand due to the overexpression of biotin receptors on certain cancer cells. These AuNPs were evaluated using a multimodality approach in vitro with three different cancer cell lines (HeLa, A549, MG63), while fibroblasts (NIH3T3) were used as controls. After treatment with AuNPs for 24 hours, the cancer cells had half the viability rate when compared to the control cells (Figure 5B). The multiplexed AuNPs showed higher internalization, visualized by dark field microscopy and confirmed by Rhodamine B fluorescence, in all three cancer cell lines than in the control cells (Figure 5C). This is a multifunctional AuNP platform that allows concurrent therapy and diagnosis, i.e. theranostics.

Figure 5
A) Schematic depiction of gold nanoparticles functionalized with biotin, Rhodamine and paclitaxel (PTX) for cancer therapy. B) Cell mortality upon incubation with functionalized AuNPs. C) Dark field microscopy images of cells incubated with AuNPs. Figure ...

Paciotti et al. developed a formulation of AuNPs, which are coated with a mixture of tumor necrosis factor (TNF) and PEG.188 TNF has potent anti-tumor effects, but its use as a therapeutic is limited by its systemic toxicity. The authors hypothesized that use of a nanoparticle delivery system could reduce the systemic toxicity of TNF and increase its tumor accumulation. Investigations of the AuNP-TNF formulation in tumor bearing mice, found a ninefold increase in TNF accumulation in the tumor. Furthermore, reductions in tumor growth and improvements in survivability were observed. These positive results encouraged the implementation of a phase I clinical trial using AuNP-TNF in patients with advanced solid organ tumors.50 Doses up to 600 ug/m2 were tested and found to be tolerable. AuNPs were detected in biopsies taken from the tumors of the patients, but not in healthy tissue. Future clinical trials are needed to evaluate the efficacy of this therapy in patients.

Nucleic acid delivery

Gene delivery is another field where AuNPs are being explored for their therapeutic potential.189191 The versatility and multifunctionality of AuNPs has facilitated several different approaches for encapsulation and release of nucleic acids. In a relatively straightforward approach, Lee et al. used AuNRs coated with cationic phospholipids as delivery vehicles for nucleic acid cargoes. The positively charged phospholipid surface was used to attach negatively charged DNA, RNA or siRNA oligonucleotides.192 Internalization of AuNRs complexed with nucleic acids was observed with darkfield scattering microscopy.

Conde et al. have developed a complex AuNP platform for efficient RNAi delivery to silence the c-myc protooncogene.193 They used a hierarchical approach, starting with cultured human cells, through invertebrates, and vertebrate (mouse) models, to select AuNP compositions that produce the most efficient therapy. The optimal formulations contained PEG chains for increased stability, RGD targeting peptides, cell penetrating TAT peptides, and siRNA either covalently or ionically attached to the nanoparticles. An elegant example of gene silencing using AuNPs as delivery agents was presented by Shim et al., shown in Figure 6.194 The siRNA was linked with multiple AuNPs through an acid-sensitive ketal linker group, forming an aggregate (Figure 6A). The ketal linker is cleaved at low pH, releasing both the oligonucleotides and the AuNPs. Changing from an aggregate to individual particles results in a radical change in the optical properties of the AuNPs. Optical coherence tomography was used to detect this change and confirm the aggregate’s disintegration at tumor relevant pH (Figure 6B). In vitro microscopy experiments on GFP expressing cells confirmed specific gene silencing under low pH conditions (Figure 6C).194

Figure 6
Gold nanoparticles for imaging and gene silencing therapy. (A) Design of the nanoparticle. Upon hydrolysis under low pH, the nanoparticle releases the siRNA cargo. (B) Three dimensional optical coherence tomography images of the nanoparticles generated ...

The optical properties of AuNPs may also be used to trigger release of nucleic acids due to strong absorption of light and resultant heating of the nanoparticle, causing the nucleic acid-nanoparticle bonds to break. Cui et al. investigated dendrimer coated AuNRs as a delivery vehicle for brcaa1-shRNA delivery into MCF7 cancer cells. Dendrimers are often used in delivery systems to increase the biocompatibility and cellular uptake. Near infrared laser irradiation triggered RNA release from AuNRs encapsulated in dendrimers that led to successful silencing of brcaa1 gene in MCF7 cells.195 Furthermore, Wijaya et al. demonstrated the conjugation of two different DNA oligonucleotides onto the surface of AuNRs with different aspect ratios. Selective release of the DNA absorbed onto the AuNRs was accomplished with irradiation at wavelengths corresponding to the characteristic absorption band of the different AuNRs.196

Photothermal therapy

The property of photon absorption and conversion into thermal energy by AuNPs quickly found use in photothermal therapies, especially for cancer applications. The irradiation of AuNPs with light of the correct wavelength induces localized temperature increases, which leads to photothermal ablation of cells in the vicinity, as such temperature increases cause biomolecule denaturation and cell damage.30, 197202 This therapeutic approach can be highly targeted to cancer due to greater nanoparticle accumulation in tumors compared with normal tissue and selective irradiation of only the tumors. For example Chen et al. synthesized Au nanocages with tunable NIR absorption between 600–1200 nm.203 In a proof of principle experiment, they showed that laser irradiation of a 1ppm solution of nanocages raised water temperature by 5–10 °C, which could lead to an increase in tissue temperature from 37 to 42 °C or greater. In vivo thermography imaging of mice bearing tumors and injected with Au nanocages indicated tumor heating post laser irradiation. Nuclear imaging indicated a therapeutic effect in terms of a decrease in the metabolism in the tumors of the injected animals. Furthermore, histology revealed cell damage in the form of coagulative necrosis after laser irradiation treatment. In another approach, Wang et al. developed supramolecular assemblies of Au-NPs (Au-SA-NPs) to collectively enhance the photothermal power in the tumor cell vicinity.204 The nanoparticles were formed from three building blocks: adamantine-grafted 2 nm Au colloids, β-cyclodextrin-grafted branched polyethylenimine, and Ad-grafted PEG. As β-cyclodextrin binds adamantine strongly, combining these building blocks led to the formation of stable Au-SA-NPs and further functionalization with the RGD peptide enabled targeting of the αvβ3 integrin receptors overexpressed on cancer cells. When applied in vitro, RGD-targeted Au-SA-NPs effectively homed to αvβ3 positive U87 glioblastoma cells, as confirmed by TEM imaging, and when the cells were irradiated with a pulsed laser the resultant was cell damage confined to the irradiation area.

Von Maltzahn et al used a theranostic approach to photothermal therapy using AuNRs to treat cancer in vivo (Figure 7A).54 The AuNRs were coated with PEG, had long circulation times of ~ 17 hours and were found to have high light absorption, about 6 times higher than Au nanoshells. CT imaging was used to generate maps of AuNR tumor accumulation post-injection into tumor bearing mice, as shown in Figure 7B. This information enabled computational modeling of the laser irradiation needed to ablate the tumor. Increases in tumor temperature were found post-irradiation (Figure 7C). Compared to controls, this regimen was much more potent, leading to almost complete tumor ablation in vivo (Figure 7D).54 In another approach, Park et al. enhanced photothermal therapy by combining the anti-cancer drug doxorubicin and gold into the same platform.205 These Au coated polymeric nanoparticles loaded with doxorubicin were used in laser irradiation experiments with HeLa cells, leading to heat-induced drug release and enhanced anticancer effects.

Figure 7
AuNRs used for both imaging and therapy. A) Schematic depiction of photothermal heating of AuNRs. B) A three dimensional rendering of CT images of a mouse bearing two tumors on its chest, which had been injected with AuNRs. The location of the AuNRs is ...

The success of these approaches has led to evaluation of AuNPs induced photothermal therapy in clinical trials. A gold nanoshell formulation known as Auralase is being tested in patients with head and neck tumors.51 Furthermore, two clinical trials have been performed using AuNPs in patients with coronary artery disease.52, 53 More clinical trials will likely be initiated in this area.

Radiotherapy

Another interesting feature of AuNPs is their radiosensitizing property. Radiotherapy is widely used in cancer therapy since radiation (X-rays, γ-rays and fast-moving charged particles such as ions, electrons and protons) induces DNA damage, thus killing cancer cells. Following the absorption of X-rays by the tumor, there is a release of scattered photons and electrons, causing DNA damage.206211 Clinically, special devices are used that irradiate the tumor from a number of angles, maximizing X-ray dose in the tumor and minimizing it in healthy tissues. Lead shielding is used to further minimize dose to healthy tissue. Gold is an excellent absorber of X-ray energy and can greatly elevate the dose of absorbed irradiation, once localized in the tumor site, thus increasing the therapeutic effects of the radiation dose.

Liu et al. tested different X-ray sources on cancer cell survival with and without the presence of PEG-coated AuNPs.14 They found that survival of cells that were exposed to X-rays in the presence of Au NPs decreased in an X-ray dose dependent fashion. In another measure of the therapeutic effect, the surviving cells had distorted morphology and abnormal distribution of organelles. Hainfeld et al. evaluated 1.9 nm, PEGylated AuNPs for enhancement of radiotherapy in tumor bearing mice.212 Tumor growth in mice injected with 1.35 g Au/kg and subjected to X-ray treatment was halted, whereas tumors in mice treated with X-rays alone continued to grow, albeit slower than untreated mice. Furthermore, at one year post-treatment, 86% of mice that were injected with 2.7 g Au/kg and irradiated with X-rays were still alive, a greatly increased percentage compared with controls.

In an alternative approach, AuNPs coated in ascorbic acid and formed from Au-198 salts were originally investigated for their potential as radiotherapeutics in the 1950s and 1960s.47, 213215 The aim in this area is that the nanoparticles will preferentially accumulate in the tumor and emit radiation which will be absorbed by the cancer cells, killing them. With the advent of improved nanoparticle synthesis methods and coating materials, this approach has been recently revived.215 Radioactive AuNP were synthesized with an epigallocatechin-gallate coating.47 Epigallocatechin-gallate was used as it has been shown to target prostate cancer. Dark field microscopy and transition electron microscopy were used to confirm that the particles were taken up in prostate cancer cells. When these nanoparticles were injected into the tumors of mice, the tumor growth was arrested over the next 35 days, indicating the therapeutic effect.

DIAGNOSTICS

AuNP have interesting applications in the molecular diagnostic field.216 The development of methods for earlier detection is important to reduce the impact of diseases and to improve survival rates. AuNPs have high surface-to-volume ratios and can be functionalized to detect specific targets, offering lower detection limits and higher selectivity than conventional strategies. They have been studied to detect analytes such as gases, ions, protein markers or DNA.217220

Mirkin’s group were pioneers in this field and have published widely on the use of AuNPs in molecular detection systems.218, 221223 In a seminal work, published in 1997, they reported an AuNP system, which is composed of two populations of AuNPs.224 Each type of AuNP is coated with different thiol-oligonucleotides. The last 15 nucleotides are complementary to one half of a target DNA sequence. When the target DNA is introduced into the system, the two types of AuNPs both bind to the DNA and aggregate. This results in a color change from red to blue, which is a well-known behavior of AuNPs. This colorimetric, AuNP-based technology has been developed into diagnostic products that are now FDA-approved. These products are used to test for Warfarin metabolism and for F5/F2/MTHFR mutations,48, 49 amongst other applications.

This methodology has been extended in a number of different directions, for example, via conductivity-based detection of DNA using microelectrodes and Ag deposition amplification of the signal arising from the AuNPs.225 Gold microelectrodes with 20 μm gaps were fabricated on a Si substrate were modified with “capture” oligonucleotide strands. The sensor was then exposed to “target” oligonucleotides in solution. The capture oligonucleotides contained short DNA sequences complementary to part of the target oligonucleotides, so upon binding overhanging strands are left exposed. This design allowed binding of AuNPs capped with oligonucleotides complementary to the target’s overhangs, bridging the electrode gaps and decreasing the resistance across the microelectrode. Ag reduction onto the AuNPs closed the spaces between the nanoparticles and the electrodes, leading to further sharp decreases in resistance. The proposed sensor allowed for the detection of target DNA in the 50 nM to 500 fM concentration range.

Conde et al. recently reported the first quantification of mutation expression in mRNA taken directly from cancer cells, using oligonucleotide coated AuNPs.226 No amplification of the RNA was needed. The AuNPs were modified with DNA complementary to the BCR-ABL b3a2 fusion transcript mRNA, which is responsible for chronic myeloid leukemia (CML). 13 nm AuNPs were modified with thiolated oligonucleotides and exposed to total RNA isolated from several cell lines. In the absence of the target DNA, the nanoparticles did not aggregate and remained red in color. When exposed to RNA extracted from cells expressing the mutant gene, there was a visible color change, enabling easy colorimetric detection.

In another approach, Lou et al. constructed a “chemical nose” sensor based on the poly(p-phenyleneethynylene) (PPE) polymer and AuNPs, capable of distinguishing 7 different proteins.217 The nanosensor was composed of an array of six AuNPs with different cationic coatings, each complexed with negatively charged PPE-CO2 polymer. The PPE polymer is highly fluorescent but the fluorescence is quenched when bound to the AuNPs. The differing capping ligands used to coat the AuNP surface, provide weaker or stronger interactions with a polymer and protein analytes. Addition of protein analytes disrupts the assembly between the AuNPs and PPE-CO2 polymer resulting in fluorescence from the polymer. The protein analytes were chosen to have different sizes and charges and thus had differential binding to the AuNPs. Therefore each protein resulted in a unique fluorescent pattern from the array, enabling their distinction in a mixture. The array was tested against 52 protein samples and correctly identified the protein with 94.2% accuracy. This approach is an excellent example of exploitation of the tunability of AuNP surface chemistry to optimize performance.

CLINICAL TRIALS AND FDA APPROVAL

From the above it is clear that there has been tremendous progress in the development of AuNP for biomedical applications. The AuNP that have been approved for clinical use are used in diagnostic applications.48, 49 The requirements for regulatory approval of diagnostic systems are relatively low, as samples are analyzed ex vivo. We expect that more such diagnostic tests will be approved in the near future. The requirements for FDA-approval of injectable AuNP are much higher, as evidenced by the relatively small number of clinical trials involving AuNP to date.5053 Nanoparticles are currently treated by the FDA in the same way as any other drug or imaging agent. Despite the relatively biocompatibility of gold, long-term retention of a large quantity of the injected material would likely prevent FDA-approval due to concerns over the long-term effects. Hence studying the excretion of AuNP is a key step towards a clinical trial. In applications where large doses are needed, such as CT, where doses as high 1.35 g Au/kg have been used,86 this is absolutely crucial. In applications where the dose of gold is much lower, such as photoacoustic imaging (as low as 22.7 μg Au/kg227), excretion may be less crucial, as small amounts of gold are typically present and tolerated in the body.228 Much of the work on AuNP for biomedical applications has arisen from the laboratories of scientists originally trained as chemists and who have developed the synthesis or studied the properties of AuNP.12, 174, 223 Translation of more AuNP to the clinic will be facilitated by closer interactions between the physical scientists who are experts on AuNP and biologists and clinicians. This will drive the design of AuNP that closely address clinical problems and the biological basis of diseases.

CONCLUSIONS

AuNPs have versatile physical properties that make them suitable for many biomedical applications. Compared with small molecules, their detectability for imaging techniques can be several orders of magnitude higher, greatly lowering detection limits. Some clinical imaging techniques including CT would benefit greatly from new nanoparticle-based contrast agents that offer longer circulation times and localized accumulation at the disease site for improved diagnoses. Also, AuNP-based platforms can be used to enhance or enable a wide variety of therapies, such as drug delivery, nucleic acid delivery, photothermal ablation and radiotherapy. The ability to tune the size, shape and consequently the physical properties of AuNPs, along with their low cytotoxicity, high biocompatibility, and range of surface chemistries makes them promising candidates for clinical use. This is borne out by AuNP-based diagnostic products now being available and FDA-approved and a number of formulations in clinical trials as therapeutics.

Table 1
Summary of gold nanoparticle imaging applications, the properties required, the suitable gold nanoparticle types and pertinent references.
Table 2
Summary of gold nanoparticle therapy applications, the properties required, the suitable gold nanoparticle types and pertinent references.
Table 3
Summary of gold nanoparticle diagnostics applications, the properties required, the suitable gold nanoparticle types and pertinent references.

Acknowledgments

This work was supported by the National Heart, Lung, and Blood Institute, National Institutes of Health, as a Program of Excellence in Nanotechnology (PEN) Award, Contract #HHSN268201000045C, as well as by R01 EB009638 (Z.A.F.), R01 CA155432 (W.J.M.M.) and R00 EB012165 (D.P.C.).

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