Magnetic nanoparticles have been extensively explored as a versatile platform for magnetic resonance imaging (MRI) contrast agents due to their strong contrast enhancement effects together with the platform capability for multiple imaging modalities. In this tutorial review, we focus on recent progress in the use of magnetic nanoparticles for MRI contrast agents and multi-mode imaging agents such as T1-T2 MRI, MRI-optical, and MRI-radioisotopes. This review also highlights emerging magnetic imaging techniques such as magnetic particle imaging (MPI), magneto-motive ultrasound imaging (MMUS), and magneto-photoacoustic imaging (MPA).

In this paper, we describe resistive-pulse sensing of two large DNAs, a single-stranded phage DNA (7250 bases) and a double-stranded plasmid DNA (6600 base pairs), using a conically shaped nanopore in a track-etched polycarbonate membrane as the sensing element. The conically shaped nanopore had a small-diameter (tip) opening of 40 nm and a large-diameter (base) opening of 1.5 microm. The DNAs were detected using the resistive-pulse, sometimes called stochastic sensing, method. This entails applying a transmembrane potential difference and monitoring the resulting ion current flowing through the nanopore. The phage DNA was driven electrophoretically through the nanopore (from tip to base), and these translocation events were observed as transient blocks in the ion current. We found that the frequency of these current-block events scales linearly with the concentration of the DNA and with the magnitude of the applied transmembrane potential. Increasing the applied transmembrane potential also led to a decrease in the duration of the current-block events. We also analyzed current-block events for the double-stranded plasmid DNA. However, because this DNA is too large to enter the tip opening of the nanopore, it could not translocate the pore. As a result, much shorter duration current-block events were observed, which we postulate are associated with bumping of the double-stranded DNA against the tip opening.

We sought to produce dendrimers conjugated to different biofunctional moieties (fluorescein [FITC] and folic acid [FA]), and then link them together using complementary DNA oligonucleotides to produce clustered molecules that target cancer cells that overexpress the high-affinity folate receptor. Amine-terminated, generation 5 polyamidoamine (G5 PAMAM) dendrimers are first partially acetylated and then conjugated with FITC or FA, followed by the covalent attachment of complementary, 5'-phosphate-modified 34-base-long oligonucleotides. Hybridization of these oligonucleotide conjugates led to the self-assembly of the FITC- and FA-conjugated dendrimers. In vitro studies of the DNA-linked dendrimer clusters indicated specific binding to KB cells expressing the folate receptor. Confocal microscopy also showed the internalization of the dendrimer cluster. These results demonstrate the ability to design and produce supramolecular arrays of dendrimers using oligonucleotide bridges. This will also allow for further development of DNA-linked dendrimer clusters as imaging agents and therapeutics.

In this review we consider recent results from our group that are directed towards developing "smart" synthetic nanopores that can mimic the functions of biological nanopores (transmembrane proteins). We first discuss the preparation and characterization of conical nanopores synthesized using the track-etch process. We then consider the design and function of conical nanopores that can rectify the ionic current that flows through these pores under an applied transmembrane potential. Finally, two types of sensors that we have developed with these conical nanopores are described. The first sensor makes use of molecular recognition elements that are bound to the nanopore mouth to selectively block the nanopore tip, thus detecting the presence of the analyte. The second sensor makes use of conical nanopores in a resistive-pulse type experiment, detecting the analyte via transient blockages in ionic current.

The authors describe efficient patterning of transparent, conductive single-walled carbon nanotube thin films by photolithography and e-beam lithography followed by reactive ion etching, and study the transport characteristics of the films patterned down to 200nm lateral dimensions. The resistivity of the films is independent of device length, while increasing over three orders of magnitude compared to the bulk films, as their width and thickness shrink. This behavior is explained by a geometrical argument. Such “top-down” patterning of nanotube films should permit their integration into submicron device structures; however, the strong resistivity scaling will have to be taken into account.

A novel nanostructure was constructed using two different generations of polyamidoamine (PAMAM) dendrimers and three sets of complementary oligonucleotides (34, 50, and 66 bases in length). The oligonucleotides were covalently conjugated to partially acetylated generation 5 and 7 PAMAM dendrimers, and these conjugates were characterized by agarose gel electrophoresis. The agarose gel electrophoresis appearance of these covalently linked oligonucleotide dendrimers was also compared to electrostatically bound oligonucleotide−dendrimer complexes. Equimolar amounts of the G5 and G7 conjugates were then hybridized together to allow for the DNA-directed self-assembly of supramolecular clusters. Dynamic light scattering (DLS) analysis indicated that the overall size of the DNA-linked dendrimer clusters tended to increase according to the length of the oligonucleotide used ranging from 30 to 50 nm, which agreed with the diameter of dendrimer nanoclusters predicted by molecular modeling. The DNA-linked novel dendrimer nanoclusters were also examined with tapping-mode atomic force microscopy (AFM) to distinguish the DNA-linked structure from a nonlinked simple G7/G5 dendrimer mixture. AFM image analysis suggested that the distance between the DNA-linked dendrimers was significantly larger than what was seen after simple mixing of G7/G5 dendrimers. The mixture showed a few dendrimers physically in contact with an interdendrimer distance of 8−10 nm. The interdendrimer distance of the nanoclusters linked with the 50-base-long oligonucleotide pairs was measured to be 21 ± 2 nm, which is in agreement with the theoretical length of the oligonucleotides duplex. These results suggest that PAMAM dendrimers can be self-assembled via complementary oligonucleotides to form supramolecular nanoclusters.

Novel QD-DNA complexes are prepared by simple electrostatic interaction between pegylated amine-functionalized CdSe/ZnS quantum dots (QDs) and DNA. The cationic nature of the amine functionality on the QD surface allows for formation of an electrostatic complex with negatively charged DNA. The presence of polyethylene glycol (PEG5000) molecules on the QD leads to enhanced stability and decreased nonspecific adsorption of DNA on the QD surface. Unlike assembly of QD-DNA based on hydrogen bonding, the present QD probes tend to be more strongly stabilized during the hybridization process by increasing the overall negative charges. In addition, the DNA loading efficiency can be modulated by changing the pH of the reaction medium. The fluorescence of the QD is quenched up to 90% by complexation with 5'-TAMRA-modified oligonucleotide (TAMRA=carboxytetramethylrhodamine) through fluorescence resonance energy transfer (FRET). With the FRET pair we selected, the R(0) value was calculated to be 5.5 nm and r is about 5 nm. This quenching of QD fluorescence is then reversed on binding of unlabeled target DNA. The maximum recovery of QD fluorescence is 60%. The QD-DNA probe (5DNA/QD) exhibits selective photoluminescence (PL) recovery in the presence of target oligonucleotide with a PL ratio of 3 for complementary versus noncomplementary. The present QD-DNA probes also show the capability to detect the synthetic 100-mer oligonucleotide derived from H5N1 influenza virus when present at concentrations as low as 200 nM in the solution.

Synthesis of biologically active antibody conjugated quantum dots (QDs) has been of great importance in cellular imaging and diagnostics. Cetuximab (or Erbitux) is the first monoclonal antibody drug which targets the epidermal growth factor receptor (EGFR) overexpressed in most cancer cells. In the present work, we investigated three different conjugation strategies to obtain the biologically functional QD-cetuximab conjugates for the tumor-specific imaging. Successful conjugation of cetuximab to QDs was achieved using PEG conjugated polymer-coated QDs and two long-chain heterobifunctional linkers, sulfo-LC-SPDP and sulfo-SMCC. The dissociation constant of the QD-cetuximab conjugates to EGFR was determined to be 0.61 +/- 0.28 nM. The cancer cell-specific binding ability of the QD-cetuximab conjugates was evaluated in vitro, and the cellular internalization of the QD-cetuximab conjugates was clearly demonstrated in live cells by confocal microscopy. The cellular imaging experiments using the QD-cetuximab conjugates showed a clear endocytosis pathway, which was evidenced by the colocalization of the QD-cetuximab conjugates with dye-labeled transferrin. These results suggest that the QD-cetuximab conjugates as an imaging modality for tumor EGFR overexpression can be expected to provide important information on the expression levels of EGFR on the cancer cells.