Targeted Positioning of Quantum Dots Inside 3D Silicon Photonic Crystals Revealed by Synchrotron X-ray Fluorescence Tomography.

It is a major outstanding goal in nanotechnology to precisely position functional nanoparticles, such as quantum dots, inside a three-dimensional (3D) nanostructure in order to realize innovative functions. Once the 3D positioning is performed, the challenge arises how to nondestructively verify where the nanoparticles reside in the 3D nanostructure. Here, we study 3D photonic band gap crystals made of Si that are infiltrated with PbS nanocrystal quantum dots. The nanocrystals are covalently bonded to polymer brush layers that are grafted to the Si-air interfaces inside the 3D nanostructure using surface-initiated atom transfer radical polymerization (SI-ATRP). The functionalized 3D nanostructures are probed by synchrotron X-ray fluorescence (SXRF) tomography that is performed at 17 keV photon energy to obtain large penetration depths and efficient excitation of the elements of interest. Spatial projection maps were obtained followed by tomographic reconstruction to obtain the 3D atom density distribution with 50 nm voxel size for all chemical elements probed: Cl, Cr, Cu, Ga, Br, and Pb. The quantum dots are found to be positioned inside the 3D nanostructure, and their positions correlate with the positions of elements characteristic of the polymer brush layer and the ATRP initiator. We conclude that X-ray fluorescence tomography is very well suited to nondestructively characterize 3D nanomaterials with photonic and other functionalities.

Selective uptake of thorium(IV) from nitric acid medium using two extraction chromatographic resins based on diglycolamide-calix[4]arenes: Application to thorium-uranyl separation in an actual sample.

Two novel extraction chromatography resins (ECRs) containing two diglycolamide (DGA) -functionalized calix[4]arenes with n-propyl and isopentyl substituents at the amide nitrogen atom, termed as ECR-1 and ECR-2, respectively, were evaluated for the uptake of Th(IV) from nitric acid feed solutions. While both the resins were having a quite high Th(IV) uptake ability (Kd >3000 at 3 M HNO3), the uptake was relatively lower with the resin containing the isopentyl DGA, which appeared magnified at lower nitric acid concentrations. Kinetic modeling of the sorption data suggested fitting to the pseudo-second order model pointing to a chemical reaction during the uptake of the metal ion. Sorption isotherm studies were carried out showing a good fitting to the Langmuir and D-R isotherm models, suggesting the uptake conforming to monolayer sorption and a chemisorption model. Glass columns with a bed volume of ca. 2.5 mL containing ca. 0.5 g lots of the ECRs were used for studies to assess the possibility of actual applications of the ECRs. Breakthrough profiles obtained with feed containing 0.7 g/L Th(NO3)3 solution resulted in breakthrough volumes of 8 and 5 mL, respectively, for the ECR-1 and ECR-2 resins. Near quantitative elution of the loaded metal ion was possible using a solution of oxalic acid and nitric acid. A method for the separation of Th-234 from natural uranium was demonstrated for the possible application of ECR-1.

Detection of Tumor DNA in Human Plasma with a Functional PLL-Based Surface Layer and Plasmonic Biosensing.

Standard protocols for the analysis of circulating tumor DNA (ctDNA) include the isolation of DNA from the patient's plasma and its amplification and analysis in buffered solutions. The application of such protocols is hampered by several factors, including the complexity and time-constrained preanalytical procedures, risks for sample contamination, extended analysis time, and assay costs. A recently introduced nanoparticle-enhanced surface plasmon resonance imaging-based assay has been shown to simplify procedures for the direct detection of tumor DNA in the patient's plasma, greatly simplifying the cumbersome preanalytical phase. To further simplify the protocol, a new dual-functional low-fouling poly-l-lysine (PLL)-based surface layer has been introduced that is described herein. The new PLL-based layer includes a densely immobilized CEEEEE oligopeptide to create a charge-balanced system preventing the nonspecific adsorption of plasma components on the sensor surface. The layer also comprises sparsely attached peptide nucleic acid probes complementary to the sequence of circulating DNA, e.g., the analyte that has to be captured in the plasma from cancer patients. We thoroughly investigated the contribution of each component of the dual-functional polymer to the antifouling properties of the surface layer. The low-fouling property of the new surface layer allowed us to detect wild-type and KRAS p.G12D-mutated DNA in human plasma at the attomolar level (∼2.5 aM) and KRAS p.G13D-mutated tumor DNA in liquid biopsy from a cancer patient with almost no preanalytical treatment of the patient's plasma, no need to isolate DNA from plasma, and without PCR amplification of the target sequence.

Increasing the Sensitivity of Electrochemical DNA Detection by a Micropillar-Structured Biosensing Surface.

The available active surface area and the density of probes immobilized on this surface are responsible for achieving high specificity and sensitivity in electrochemical biosensors that detect biologically relevant molecules, including DNA. Here, we report the design of gold-coated, silicon micropillar-structured electrodes functionalized with modified poly-l-lysine (PLL) as an adhesion layer to concomitantly assess the increase in sensitivity with the increase of the electrochemical area and control over the probe density. By systematically reducing the center-to-center distance between the pillars (pitch), denser micropillar arrays were formed at the electrode, resulting in a larger sensing area. Azido-modified peptide nucleic acid (PNA) probes were click-reacted onto the electrode interface, exploiting PLL with appended oligo(ethylene glycol) (OEG) and dibenzocyclooctyne (DBCO) moieties (PLL-OEG-DBCO) for antifouling and probe binding properties, respectively. The selective electrochemical sandwich assay formation, composed of consecutive hybridization steps of the target complementary DNA (cDNA) and reporter DNA modified with the electroactive ferrocene functionality (rDNA-Fc), was monitored by quartz crystal microbalance. The DNA detection performance of micropillared electrodes with different pitches was evaluated by quantifying the cyclic voltammetric response of the surface-confined rDNA-Fc. By decrease of the pitch of the pillar array, the area of the electrode was enhanced by up to a factor 10.6. A comparison of the electrochemical data with the geometrical area of the pillared electrodes confirmed the validity of the increased sensitivity of the DNA detection by the design of the micropillar array.

"Plug-n-Play" Polymer Substrates: Surface Patterning with Reactive-Group-Appended Poly-l-lysine for Biomolecule Adhesion.

The immobilization of biomolecules onto polymeric surfaces employed in the fabrication of biomedical and biosensing devices is generally a challenging issue, as the absence of functional groups in such materials does not allow the use of common surface chemistries. Here we report the use of modified poly-l-lysine (PLL) as an effective method for the selective modification of polymeric materials with biomolecules. Cyclic olefin polymer (COP), Ormostamp, and polydimethylsiloxane (PDMS) surfaces were patterned with modified PLLs displaying either biotin or maleimide functional groups. Different patterning techniques were found to provide faithful microscale pattern formation, including micromolding in capillaries (MIMIC) and a hydrogel-based stamping device with micropores. The surface modification and pattern stability were tested with fluorescence microscopy, contact angle and X-ray photoelectron spectroscopy (XPS), showing an effective functionalization of substrates stable for over 20 days. By exploiting the strong biotin-streptavidin interaction or the thiol-maleimide coupling, DNA and PNA probes were displayed successfully on the surface of the materials, and these probes maintained the capability to specifically recognize complementary DNA sequences from solution. The printing of three different PNA-thiol probe molecules in a microarray fashion allowed selective DNA detection from a mixture of DNA analytes, demonstrating that the modified PLL methodology can potentially be used for multiplexed detection of DNA sequences.

Control of Probe Density at DNA Biosensor Surfaces Using Poly(l-lysine) with Appended Reactive Groups.

Biosensors and materials for biomedical applications generally require chemical functionalization to bestow their surfaces with desired properties, such as specific molecular recognition and antifouling properties. The use of modified poly(l-lysine) (PLL) polymers with appended oligo(ethylene glycol) (OEG) and thiol-reactive maleimide (Mal) moieties (PLL-OEG-Mal) offers control over the presentation of functional groups. These reactive groups can readily be conjugated to, for example, probes for DNA detection. Here we demonstrate the reliable conjugation of thiol-functionalized peptide nucleic acid (PNA) probes onto predeposited layers of PLL-OEG-Mal and the control over their surface density in the preceding synthetic step of the PLL modification with Mal groups. By monitoring the quartz crystal microbalance (QCM) frequency shifts of the binding of complementary DNA versus the density of Mal moieties grafted to the PLL, a linear relationship between probe density and PLL grafting density was found. Cyclic voltammetry experiments using Methylene Blue-functionalized DNA were performed to establish the absolute probe density values at the biosensor surfaces. These data provided a density of 1.2 × 1012 probes per cm2 per % of grafted Mal, thus confirming the validity of the density control in the synthetic PLL modification step without the need of further surface characterization.

DNA Detection by Flow Cytometry using PNA-Modified Metal-Organic Framework Particles.

A DNA-sensing platform is developed by exploiting the easy surface functionalization of metal-organic framework (MOF) particles and their highly parallelized fluorescence detection by flow cytometry. Two strategies were employed to functionalize the surface of MIL-88A, using either covalent or non-covalent interactions, resulting in alkyne-modified and biotin-modified MIL-88A, respectively. Covalent surface coupling of an azide-dye and the alkyne-MIL-88A was achieved by means of a click reaction. Non-covalent streptavidin-biotin interactions were employed to link biotin-PNA to biotin-MIL-88A particles mediated by streptavidin. Characterization by confocal imaging and flow cytometry demonstrated that DNA can be bound selectively to the MOF surface. Flow cytometry provided quantitative data of the interaction with DNA. Making use of the large numbers of particles that can be simultaneously processed by flow cytometry, this MOF platform was able to discriminate between fully complementary, single-base mismatched, and randomized DNA targets.

Monolayer-directed assembly and magnetic properties of FePt nanoparticles on patterned aluminum oxide.

FePt nanoparticles (NPs) were assembled on aluminum oxide substrates, and their ferromagnetic properties were studied before and after thermal annealing. For the first time, phosph(on)ates were used as an adsorbate to form self-assembled monolayers (SAMs) on alumina to direct the assembly of NPs onto the surface. The Al(2)O(3) substrates were functionalized with aminobutylphosphonic acid (ABP) or phosphonoundecanoic acid (PNDA) SAMs or with poly(ethyleneimine) (PEI) as a reference. FePt NPs assembled on all of these monolayers, but much less on unmodified Al(2)O(3), which shows that ligand exchange at the NPs is the most likely mechanism of attachment. Proper modification of the Al(2)O(3) surface and controlling the immersion time of the modified Al(2)O(3) substrates into the FePt NP solution resulted in FePt NPs assembly with controlled NP density. Alumina substrates were patterned by microcontact printing using aminobutylphosphonic acid as the ink, allowing local NP assembly. Thermal annealing under reducing conditions (96%N(2)/4%H(2)) led to a phase change of the FePt NPs from the disordered FCC phase to the ordered FCT phase. This resulted in ferromagnetic behavior at room temperature. Such a process can potentially be applied in the fabrication of spintronic devices.

Expression of sensitized Eu(3+) luminescence at a multivalent interface.

Assembly of a mixture of guest-functionalized antenna and Eu(3+)-complexed ligand molecules in a patterned fashion onto a receptor surface provides efficient localized sensitized emission. Coordination of a carboxylate group of the antenna to the Eu(3+) center and noncovalent anchoring of both components to the receptor surface appeared to be prerequisites for efficient energy transfer. A Job plot at the surface confirmed that coordination of the antenna to the Eu(3+) center occurs in a 1:1 fashion. The efficiency of this intramolecular binding process is promoted by the high effective concentration of both complementary moieties at the surface. The system constitutes therefore an example of supramolecular expression of a complex consisting of several different building blocks which signals its own correct formation.