Exceptional magnetic properties of magnetite, Fe₃O₄, nanoparticles make them one of the most intensively studied inorganic nanomaterials for biomedical applications. We report successful gram-scale syntheses, via hydrothermal route or controlled coprecipitation in an automated reactor, of colloidal Fe₃O₄ nanoparticles with sizes of 12.9 ± 5.9, 17.9 ± 4.4, and 19.8 ± 3.2 nm. To investigate structure–property relationships as a function of the synthetic procedure, we used multiple techniques to characterize the structure, phase composition, and magnetic behavior of these nanoparticles. For the iron oxide cores of these nanoparticles, powder X-ray diffraction and electron microscopy both confirm single-phase Fe₃O₄ composition. In addition to the core composition, the magnetic performance of nanoparticles in the 13–20 nm size range can be strongly influenced by the surface properties, which we analyzed by three complementary techniques. Raman scattering and X-ray photoelectron spectroscopy (XPS) measurements indicate overoxidation of nanoparticle surfaces, while transmission electron microscopy (TEM) shows no distinct core–shell structure. Considered together, Raman, XPS, and TEM observations suggest that our nanoparticles have a gradually varying nonstoichiometric Fe₃O_4+δ composition, which could be attributed to the formation of Fe₃O₄–γ-Fe₂O₃ solid solutions at their outermost surface. Detailed analyses by TEM reveal that the hydrothermally produced samples include single-domain nanocrystals coexisting with defective twinned and dimer nanoparticles, which form as a result of oriented-attachment crystal growth. All our nanoparticles exhibit superparamagnetic-like behavior with a characteristic blocking temperature above room temperature. We attribute the estimated saturation magnetization values up to 84.01 ± 0.25 emu/g at 300 K to the relatively large size of the nanoparticles (13–20 nm) coupled with the syntheses under elevated temperature; alternative explanations, such as surface-mediated effects, are not supported by our spectroscopy or microscopy measurements. For these colloids, the heating efficiency in magnetic hyperthermia correlates with their saturation magnetization, making them appealing for therapeutic and other biomedical applications that rely on high-performance nanoparticle-mediated hyperthermia.

We describe self-assembly of ssDNA brushes that exploits the intrinsic affinity of adenine nucleotides (dA) for gold surfaces. The grafting density and conformation of these brushes is deterministically controlled by the length of the anchoring dA sequences, even in the presence of thymine nucleotides (dT). We produce and characterize brushes of model block-oligonucleotides, d(T _m -A _n ), with systematically varied lengths m and n of the thymine and adenine blocks [denoted d(T _m ) and d(A _n ), respectively]. The hairpin conformation, dominant for self-complementary d(T _m -A _n ) oligos in solution, is disrupted by the high preferential affinity of dA for gold surfaces. As a result, the d(T _m -A _n ) oligos adsorb as a brush of d(T) strands immobilized via the d(A) blocks. Quantitative analysis by FTIR spectroscopy and x-ray photoelectron spectroscopy (XPS) reveals a unique feature of DNA immobilization via d(A) blocks: The surface density of dA nucleotides is close to saturation and is nearly independent of d(A) block length. Accordingly, the lateral spacing (grafting density) of the d(T) blocks is determined by the length of the d(A) blocks. The d(T) blocks extend away from the surface in a brush-like conformation at a lateral spacing 2–3 times larger (a grafting density 5–10 times lower) than in analogous films immobilized via standard thiol linkers. This combination of brush-like conformation and low saturation grafting density is expected to increase the efficiency of DNA hybridization at surfaces. Therefore, immobilization via d(A) blocks offers a method of producing DNA brushes with controlled properties for applications in biotechnology and nanotechnology.