Multifunctional mesoporous silica nanoparticles (MSNs) have aroused much attention during the past decades for drug delivery and bioimaging applications because of their intrinsic properties including extremely high surface area, large pore volume, tunable pore diameter, easy surface modification, and high biocompatibility. Even though MSNs have these preeminent properties, rendering this unique nanostructure a promising nanocarrier for biomedical applications, several hurdles still challenge the fields of drug delivery and bioimaging when using MSNs as the nanocarries: (i) high loading and high release amounts of water-insoluble drugs delivered to the site of diseases; (ii) precise control of the dosage of drugs delivered to the site of diseases using non-invasive external stimuli; and (iii) construction of MSNs-based shortwave infrared optical imaging contrast agents as an innovative tool for bioimaging and cancer diagnostics. Therefore, this dissertation primarily focuses on the development of innovative strategies that solve these unmet needs and that advance the research in the field of biomedical applications using MSNs as the nanocarriers.

In this dissertation, first of all, we review the research work, which mainly focuses on the design and synthesis of multifunctional MSNs and nanomachines for biomedical applications in Accounts of Chemical Research. A wide variety of nanomachines responsive to the different stimuli (pH, redox, enzyme, heat, light, and/or magnetic field) are discussed in this Account. Additionally, we develop a facile strategy for MSNs delivery and release of the water-insoluble drug clofazimine (CFZ), which is used to treat multidrug-resistant tuberculosis. The strategy employs a companion molecule as a chaperone to improve both the loading of CFZ into the pores of MSNs and its subsequent release, thus enabling both high loading and high release of this water-insoluble drug by MSNs. In vitro treatment of macrophages infected with Mycobacterium tuberculosis with the optimized CFZ-loaded MSNs killed the bacteria in the cells in a dose-dependent manner. These studies demonstrate a highly efficient method for loading nanoparticles with water-insoluble drug molecules and the efficacy of the nanoparticles in delivering drugs into eukaryotic cells in aqueous media. Additionally, we used a noninvasive alternating magnetic field (AMF) to stimulate and control the dosage of drug release from MSNs. Noninvasive stimuli-responsive drug delivery using AMF in conjunction with superparamagnetic nanoparticles also offers the potential for the spatial and temporal control of drug release. In vitro studies showed that the death of pancreatic cancer cells treated by drug-loaded nanoparticles was controlled by different lengths of AMF exposure time due to different amounts of drug released from the carriers. Finally, to develop a new shortwave infrared (SWIR) optical imaging contrast agent which has a higher tissue penetration depth, we demonstrate that J-aggregates of near infrared (NIR) fluorophore IR-140 can be prepared inside hollow mesoporous silica nanoparticles (HMSNs) to result in nanomaterials that absorb and emit SWIR light. The use of J-aggregates stabilized in HMSNs as SWIR imaging agents has the potential to overcome the stability, toxicity, and brightness challenges of contrast agents for this compelling region of the electromagnetic spectrum. Collectively, in this dissertation, we explore and develop innovative strategies to load and deliver high amounts of water-insoluble drugs; control the dosage of anticancer drugs released from MSNs triggered by an AMF; and establish a new SWIR optical imaging contrast agent based on the superior carriers – MSNs.

The on-demand drug release of mesoporous silica nanoparticles is investigated by chemical modifications of nanomachines and by spectroscopic examination of their physical properties. To improve and diversify their biological performances, acid responsive nanovalve release systems are integrated with gadolinium MRI contrast agents and polyethylene imine-polyethylene glycol surface coatings. Nanoparticles, and bulk media for comparison, are heated superparamagnetically and their temperatures are monitored using the luminescence spectra of lanthanide upconversion nanocrystals. This thermometry technique is applied again in azobenzene nanoimpeller particles to analyze the photothermal and photochemical contributions of IR radiation in initiating cargo release. The emission spectra prove capable of revealing the nanoscale temperature change and the macroscopic dye release simultaneously. Mesoporous silica nanoparticles demonstrate versatile nanomedical delivery capabilities in vitro and in vivo.

This dissertation makes contributions to the fields of functionalized mesoporous silica nanoparticles (MSNs) designed for magnetic resonance imaging (MRI) contrast enhancement and high-intensity focused ultrasound (HIFU) triggered cargo release. MRI-guided HIFU (MRIgHIFU) has been applied as a therapeutic tool in the clinic, and enhanced MRI contrast will improve the precision and applicability of HIFU therapy. In Chapter 2, the ultrasound responsive MSNs carriers are reviewed. In Chapter 3, the proof-of-concept spotlight technique based on MRIgHIFU and MSNs is introduced. With periodic HIFU modulation, the functionalized MSNs generated reversible MRI T1 relaxivity changes at the 1.5 mm3 focal point. Fourier analysis was used to extract signal changes at the modulation frequency, and a modulation enhancement map spotlights the precise region of interest by increasing contrast almost 100-fold. In Chapter 4, another functionalized MSNs were designed for the spotlight technique, and the spectral analysis was further studied to maximize the contrast enhancement. In particular, a framework was presented to analyze the trade-offs between different parameter choices for the signal processing method. In Chapter 5, the application of the spotlight technique was demonstrated at human body temperature. The MSNs were functionalized with the tailored poly(2-oxazoline)s with a lower critical solution temperature (LCST) at 40 �C, so that it could generate reversible MRI contrast change at human body temperature. The temperature oscillation range was small (3 �C), and the data acquisition was relatively fast (in 100 s), which made it a step closer to the biomedical applications in the clinic. The HIFU-responsive cargo delivery was studied in Chapters 6 and 7. The large pore MSNs (LPMSNs) were studied as the nanocarriers to load a large drug molecule, Docetaxel. With the loading procedure and HIFU parameters tuned, the HIFU-responsive release was demonstrated in a biologically relevant solvent. HIFU-responsive MRI contrast agent delivery was studied using MSNs capped with thermo-responsive polymer. The cargo loading and release conditions were optimized, and the HIFU-responsive cargo release can be monitored by MRI contrast change. The spotlight technique in this dissertation is promising for biomedical applications of precision therapy. The framework on parameter choice and demonstrated MRI contrast enhancement at human body temperature provides the foundation for further clinical translation.

This dissertation makes contributions to the fields of anticancer drug formulation, synthesis and applications of nanoparticle adjuvants, and structure-activity relationships in drug discovery. Many drugs that are promising clinical candidates are potent in vitro, however their solubility and pharmacokinetic properties are suboptimal. Here we used various nano-formulations, primarily mesoporous silica nanoparticles, to improve the solubility and release properties of these drugs. In vivo results show that these nanoparticles are safe, multifunctional carriers that can be used for clinical imaging using positron emission tomography. In the second part of this work, porous aluminum oxide nanoparticles were synthesized for applications as immunological adjuvants, and polymer-coated mesoporous silica nanoparticles were used to deliver cyclic dinucleotides to stimulate the innate immune response. Lastly, UV-Vis spectroscopic assays were developed and used to understand how electronic properties of iron-binding heterocyclic thiosemicarbazone analogs are related to their inhibition of ribonucleotide reductase, an important target for cancer therapy. The structure-activity relationships that were discovered led to a promising drug scaffold for the treatment of cancers sensitive to nucleotide deprivation.

In the recent biomedical research, mesoporous silica nanoparticles (MSNs) prepared by sol-gel chemistry have useful applications in cancer treatment. This is because of several attractive aspects of the MSN such as uniform pores with tunable size, large surface area, ease in functionalization, and biocompatibility. Also when the pores are functionalized with organic molecules that are stimuli-responsive, a controlled release system can be obtained for storage and delivery of a wide range of drugs through the pores. Among many therapeutic MSNs, light-activated silica nanoparticles are advantageous because ease in applying light activation allows for control of the system at a distance and a desired time. MSNs discussed in this dissertation involve two different types of the photo-responsive behaviors of functional molecules: (1) controlled release of pore contents by azobenzene-based nanoimpellers, and (2) generation of reactive oxygen species (ROS) by porphyrin-based photosensitizers.

Nanoimpeller-controlled MSNs, which are a main focus of this dissertation, are discussed in Chapter 2-4. Nanoimpellers consist of photo-responsive azobenzene derivatives that are attached to the mesopores, and undergo photoiosmerization that results in dynamic wagging motions of the unbound termini and drives the expulsion of molecules from the pores. Controlled release of molecules is demonstrated in solution using fluorescent dyes (Chapter 2) and then in human cancer cells using an anticancer drug to induce cell death (Chapter 3). Various molecules having different water solubilities are tested in non-aqueous, aqueous, and intracellular environments as well (Chapter 4). The last part of the dissertation illustrates photosensitizer-functionalized MSNs for photodynamic therapy (PDT). A photosensitizer porphyrin derivative that is bonded to the pore wall is operated to generate singlet oxygen by light irradiation to treat malignant cells (Chapter 5).

The field of nanomachines based on mesoporous and microporous silica nanoparticles is a relatively new one, but has quickly gained widespread popularity due to their large potential applications. These porous nanomaterials can both carry and release a therapeutic drug molecule at a targeted location. In order to regulate the movement of cargo, nanomachines are designed and assembled onto the silica nanoparticle, ultimately creating a delivery system on the nanoscale that is capable of a stimulus-responsive delivery of its cargo.

This dissertation focuses on the design, synthesis and assembly of nanomachines on both meso- and microporous silica nanoparticles to achieve the goal of cargo delivery. The six chapters of this dissertation are presented as follows: 1) the design, synthesis and modification of silica nanoparticles for their use in biology, 2) a light activated, reversible nanovalve assembled on mesoporous silica nanoparticles to achieve a size-selective cargo delivery, 3) biological applications and the delivery of anti-cancer drugs using a pseudorotaxane-based light activated nanovalve, 4) a nanogate machine that is capable of the storage and delivery of both small metal ions and useful organic cargo molecules, 5) biological applications of the nanogate machine in order to deliver calcium ions to cancerous cells to induce cell apoptosis, and 6) thin wax coated microporous silica nanoparticles that are capable of delivering small ions including oxidizers.

This dissertation makes contributions to the fields of formulation and delivery of anticancer drugs, antibiotics, and imaging agents, primarily focused on engineering mesoporous silica nanoparticles (MSNs) for stimuli-responsive drug delivery. The strategies and techniques developed in this dissertation will be especially useful for achieving precision medicine or personalized medicine, which is defined as the “right drug, right dosage at right timing to right patient”. Although various emerging approaches for personalized disease treatment that take individual variability into account have been developed, the necessity of delivering the desired therapeutics at the desired time to the specific site of the disease and with accurate dosage remains a challenge. Here we first review the previously reported stimuli-responsive MSNs controlled by supramolecular nanomachines for antibiotic and drug delivery. In the second part of the dissertation, we report novel MSNs-based nanoparticles engineered to be responsive to noninvasive stimuli, such as alternating magnetic field (AMF) or high-intensity focused ultrasound (HIFU). AMF-responsive drug delivery demonstrates the controlled therapeutic efficacy for pancreatic cancer cells in vitro by adjusting different lengths of AMF exposure time. The HIFUresponsive MSNs provide a promising platform for magnetic resonance imaging (MRI)-guided HIFU (MRgHIFU)-stimulated cargo delivery. The change of T1 reports on the amount of released cargo which is imageable by MRI ex vivo. Both AMF-and MRgHIFU-stimulation strategies offer the potential for the spatial, temporal, and dosage control of drug delivery. In the last part, we develop an approach to achieve both high loading and high release amount of a water-insoluble antibiotic clofazimine (CFZ) carried by MSNs by using acetophenone (AP) as a chaperone molecule, solving the water insolubility problem faced when treating multidrug-resistant tuberculosis. The treatment of Mycobacterium tuberculosis infected macrophages with optimized CFZ-loaded MSNs shows good therapeutic efficacy in vitro. Finally, we develop a hollow mesoporous silica nanoparticle (HMSN) formulated near infrared (NIR) fluorophore IR-140, to realize a novel biocompatible shortwave infrared (SWIR) optical imaging contrast agent for bioimaging with a higher tissue penetration depth. The J-aggregates of IR-140 stabilized inside HMSNs showed the potential to overcome the stability, toxicity, and brightness challenges faced by common SWIR contrast agents.

The research covered in this dissertation is described in six chapters. The first two chapters focus on the design of supramolecular nanovalves on mesoporous silica and silicon materials to form drug delivery systems. These systems are capable of autonomously releasing drug molecules in vitro. The third chapter describes size-selective nanovalves that are able to control the release of both large and small molecules. These nanovalves are constructed on mesoporous silica materials with 6.5 nm pores. The fourth chapter demonstrates two experimental methods, dynamic fluorescence anisotropy and rigidochromism spectroscopy, for probing the properties of the microenvironment inside the mesopores of mesoporous silica nanoparticles. In the fifth chapter of this dissertation, the design and operation of a chemical amplifier based on enzyme-encapsulated mesoporous silica are shown. This system employs a pH-responsive nanogate assembly to control the access to the encapsulated enzyme. It is the first example of a chemical amplifier that is activated by and also amplifies the same type of reaction. The last chapter describes the spectroscopic consequences of the excited state mixed valence in a binuclear copper complex using a neighboring orbital model. The copper complex represents a new class of excited state mixed valence compounds, where the coupling of two equivalent charge-baring units is mediated through a copper-iodine cluster. The result from the simple neighboring orbital approach is supported by resonance Raman spectroscopy and Gaussian calculations.