Strand invasion of DNA quadruplexes by PNA: comparison of homologous and complementary hybridization

Molecular recognition of DNA quadruplex structures is envisioned to be a strategy for regulating gene expression at the transcriptional level and for in situ analysis of telomere structure and function. The recognition of DNA quadruplexes by peptide nucleic acid (PNA) oligomers is presented here, with a focus on comparing complementary, heteroduplex-forming and homologous, heteroquadruplex-forming PNAs. Surface plasmon resonance and optical spectroscopy experiments demonstrated that the efficacy of a recognition mode depended strongly on the target. Homologous PNA readily invades a quadruplex derived from the promoter regulatory region found upstream of the MYC proto-oncogene to form a heteroquadruplex at high potassium concentration mimicking the intracellular environment, whereas complementary PNA exhibits virtually no hybridization. In contrast, complementary PNA is superior to the homologous in hybridizing to a quadruplex modeled on the human telomere sequence. The results are discussed in terms of the different structural morphologies of the quadruplex targets and the implications for in vivo recognition of quadruplexes by PNAs.

[Affinity capture of specific DNA fragments with the use of short synthetic sequences]

The ability of short peptide nucleic acid (PNA) oligomers and oligonucleotides containing modified residues of 5-methylcitidine, 2-aminoadenosine and 5-propynyl-2'-deoxyuridine (strong binding oligonucleotides, SBO) to affinity capture the target double-stranded DNA fragment from mixture by means of the end invasion was compared. Both types of probes were highly effective at the conditions used. The SBO-based probes may represent a handy and easily prepared alternative to PNA for selection of target DNA fragments from mixtures.

Quantitative and multiplexed microRNA sensing in living cells based on peptide nucleic acid and nano graphene oxide (PANGO)

MicroRNA (miRNA) is an important small RNA which regulates diverse gene expression at the post-transcriptional level. miRNAs are considered as important biomarkers since abnormal expression of specific miRNAs is associated with many diseases including cancer and diabetes. Therefore, it is important to develop biosensors to quantitatively detect miRNA expression levels. Here, we develop a nanosized graphene oxide (NGO) based miRNA sensor, which allows quantitative monitoring of target miRNA expression levels in living cells. The strategy is based on tight binding of NGO with peptide nucleic acid (PNA) probes, resulting in fluorescence quenching of the dye that is conjugated to the PNA, and subsequent recovery of the fluorescence upon addition of target miRNA. PNA as a probe for miRNA sensing offers many advantages including high sequence specificity, high loading capacity on the NGO surface compared to DNA and resistance against nuclease-mediated degradation. The present miRNA sensor allowed the detection of specific target miRNAs with the detection limit as low as ~1 pM and the simultaneous monitoring of three different miRNAs in a living cell.

Optimized templates for bottom-up growth of high-performance integrated biomolecular detectors

Electrochemical deposition of metals represents an important approach in the bottom-up fabrication of nanostructures and microstructures. We have used this approach to generate high-performance chip-based biosensors using silicon as a platform for the generation of sensor arrays. Here, we explore the applicability of different materials to support the electrodeposition and identify the parameters that are essential for robust sensor growth. We show that inexpensive materials can be used as templates for electrodeposition, and demonstrate that these low-cost sensors exhibit clinically-relevant levels of sensitivity and specificity. In particular, we prove herein that the glass-based sensors successfully detect E. coli in urine, when present at the 100 cfu μL(-1) levels found typically in samples of patients with urinary tract infections.

The single-molecule conductance and electrochemical electron-transfer rate are related by a power law

This study examines quantitative correlations between molecular conductances and standard electrochemical rate constants for alkanes and peptide nucleic acid (PNA) oligomers as a function of the length, structure, and charge transport mechanism. The experimental data show a power-law relationship between conductances and charge transfer rates within a given class of molecules with the same bridge chemistry, and a lack of correlation when a more diverse group of molecules is compared, in contrast with some theoretical predictions. Surprisingly, the PNA duplexes exhibit the lowest charge-transfer rates and the highest molecular conductances. The nonlinear rate-conductance relationships for structures with the same bridging chemistries are attributed to differences in the charge-mediation characteristics of the molecular bridge, energy barrier shifts and electronic dephasing, in the two different experimental settings.

DISSECT Method Using PNA-LNA Clamp Improves Detection of EGFR T790m Mutation

Non-small cell lung cancer (NSCLC) patients treated with small molecule EGFR inhibitors, such as gefitinib, frequently develop drug resistance due to the presence of secondary mutations like the T790M mutation on EGFR exon 20. These mutations may originate from small subclonal populations in the primary tumor that become dominant later on during treatment. In order to detect these low-level DNA variations in the primary tumor or to monitor their progress in plasma, it is important to apply reliable and sensitive mutation detection methods. Here, we combine two recently developed methodologies, Differential Strand Separation at Critical Temperature (DISSECT), with peptide nucleic acid-locked nucleic acid (PNA-LNA) polymerase chain reaction (PCR) for the detection of T790M EGFR mutation. DISSECT pre-enriches low-abundance T790M EGFR mutations from target DNA prior to implementing PNA-LNA PCR, a method that can detect 1 mutant allele in a background of 100–1000 wild type alleles. The combination of DISSECT and PNA-LNA PCR enables the detection of 1 mutant allele in a background of 10,000 wild type alleles. The combined DISSECT-PNA-LNA PCR methodology is amenable to adaptation for the sensitive detection of additional emerging resistance mutations in cancer.

[Composites of peptide nucleic acids with titanium dioxide nanoparticles. I. Construction of nanocomposites containing DNA/PNA duplexes and their delivery into HeLa cells]

In order to investigate the possibility of using titanium dioxide (TiO2) nanoparticles to transport peptide nucleic acids (PNA) in eukaryotic cells, a PNA oligomer has been synthesized, and method of PNA immobilization in the form of hybrid DNA/PNA duplexes on the surface of TiO2 nanoparticles covered with polylysine (PL) has been designed. Attaching of DNA/PNA duplex on TiO2 x PL nanoparticles occurred due to electrostatic interactions between the negatively charged DNA chain and the positively charged amino groups of PL. Binding of the PNA with the nanocomposite achieved through noncovalent Watson-Crick interactions between the PNA and complementary DNA. The capacity of obtained TiO2 x PL x DNA/PNA nanocomposites depending on immobilization conditions was 10-30 nmol PNA per 1 mg of TiO2 particles, which corresponds to -1-3 PNA molecules per one TiO2 particle with size of 4-6 nm. By method ofconfocal laser scanning microscopy on the example of the fluorescein labeled PNA oligomer (Flu)PNA it has been shown that the PNA molecules in composition of TiO2 x PL x DNA/(Flu)PNA nanocomposites effectively penetrate and accumulate in HeLa cells without the use oftransfection agents, electroporation, or other auxiliary procedures has been shown.

Peptide nucleic acid (PNA)-DNA duplexes: comparison of hybridization affinity between vertically and horizontally tethered PNA probes

We compare the PNA-DNA duplex hybridization characteristics of vertically tethered and new horizontally tethered PNA probes on solid surfaces. The horizontal 15-mer PNA probe has been synthesized with linker molecules attached at three locations (γ-points) positioned along the PNA backbone that provides covalent attachment of the probe with the backbone aligned parallel to the surface, which is important for DNA hybridization assays that use electric field effect sensors for detection. A radioactive labeled assay and real-time surface plasmon resonance (SPR) biosensor are used to assess the probe surface density, nonspecific binding, and DNA hybridization affinity, respectively, of the new PNA probe configuration. The estimated equilibrium dissociation constants of the horizontally tethered duplex and the vertically tethered duplex are of the same order of magnitude (KD ≈ 5 nM), which indicates a sufficient hybridization affinity for many electronic biosensors that benefit from the horizontal alignment, which minimizes the effects of counterion screening.

Sequence-specific and visual identification of the influenza virus NS gene by azobenzene-tethered bis-peptide nucleic acid

To rapidly and specifically identify highly virulent influenza virus strains, we prepared an azobenzene-tethered hairpin-type peptide nucleic acid, bisPNA-AZO, which has a complementary sequence against a highly conserved genomic RNA sequence within the ribonucleoprotein complex of the 2009 pandemic influenza A virus, H1N1 subtype. bisPNA-AZO recognizes the conserved virus genome sequence in a sequence-specific manner. Immobilization of bisPNA-AZO on a plate allowed capture of the target virus gene and the generation of a visual colour signal.

Imaging mRNA expression levels in living cells with PNA·DNA binary FRET probes delivered by cationic shell-crosslinked nanoparticles

Optical imaging of gene expression through the use of fluorescent antisense probes targeted to the mRNA has been an area of great interest. The main obstacles to developing highly sensitive antisense fluorescent imaging agents have been the inefficient intracellular delivery of the probes and high background signal from unbound probes. Binary antisense probes have shown great promise as mRNA imaging agents because a signal can only occur if both probes are bound simultaneously to the mRNA target site. Selecting an accessible binding site is made difficult by RNA folding and protein binding in vivo and the need to bind two probes. Even more problematic, has been a lack of methods for efficient cytoplasmic delivery of the probes that would be suitable for eventual applications in vivo in animals. Herein we report the imaging of iNOS mRNA expression in live mouse macrophage cells with PNA·DNA binary FRET probes delivered by a cationic shell crosslinked knedel-like nanoparticle (cSCK). We first demonstrate that FRET can be observed on in vitro transcribed mRNA with both the PNA probes and the PNA·DNA hybrid probes. We then demonstrate that the FRET signal can be observed in live cells when the hybrid probes are transfected with the cSCK, and that the strength of the FRET signal is sequence specific and depends on the mRNA expression level.

Label-free DNA biosensor based on a peptide nucleic acid-functionalized microstructured optical fiber-Bragg grating

We describe a novel sensing approach based on a functionalized microstructured optical fiber-Bragg grating for specific DNA target sequences detection. The inner surface of a microstructured fiber, where a Bragg grating was previously inscribed, has been functionalized by covalent linking of a peptide nucleic acid probe targeting a DNA sequence bearing a single point mutation implicated in cystic fibrosis (CF) disease. A solution of an oligonucleotide (ON) corresponding to a tract of the CF gene containing the mutated DNA has been infiltrated inside the fiber capillaries and allowed to hybridize to the fiber surface according to the Watson-Crick pairing. In order to achieve signal amplification, ON-functionalized gold nanoparticles were then infiltrated and used in a sandwich-like assay. Experimental measurements show a clear shift of the reflected high order mode of a Bragg grating for a 100 nM DNA solution, and fluorescence measurements have confirmed the successful hybridization. Several experiments have been carried out on the same fiber using the identical concentration, showing the same modulation trend, suggesting the possibility of the reuse of the sensor. Measurements have also been made using a 100 nM mismatched DNA solution, containing a single nucleotide mutation and corresponding to the wild-type gene, and the results demonstrate the high selectivity of the sensor.

IDH1 mutations in oligodendroglial tumors: comparative analysis of direct sequencing, pyrosequencing, immunohistochemistry, nested PCR and PNA-mediated clamping PCR

Mutations in isocitrate dehydrogenase 1 (IDH1) are found in a high proportion of glial tumors and have a significant prognostic impact. Although direct sequencing has been considered to be the gold-standard method to detect this mutation, the sensitivity of this technique has been questioned especially because specimens from glial tumors may contain large numbers of non-tumor cells. We screened 141 cases of oligodendroglial tumors for IDH1 mutations using peptide nucleic acid (PNA)-mediated clamping polymerase chain reaction (PCR) and compared the results with the results of direct sequencing, pyrosequencing, and immunohistochemistry (IHC). Nested PCR was only performed in cases having mutant IDH1 only discovered by clamping PCR. Using dilution experiments mixing IDH1 wild-type and mutant DNA samples, clamping PCR detected mutations in samples with a 1% tumor DNA composition. Using PNA clamping PCR, we detected 138 of 141 (97.9%) cases with mutant IDH1 in our series, which is significantly higher (P = 0.016; PNA clamping vs. direct sequencing) than those of direct sequencing (74.5%), pyrosequencing (75.2%) and IHC (75.9%). From our results, almost all oligodendroglial tumors have IDH1 mutations, and this suggests that IDH1 mutation is an early and common event especially in the development of oligodendroglial tumors.

Development of peptide nucleic acid probes for detection of the HER2 oncogene

Peptide nucleic acids (PNAs) have gained much interest as molecular recognition tools in biology, medicine and chemistry. This is due to high hybridization efficiency to complimentary oligonucleotides and stability of the duplexes with RNA or DNA. We have synthesized 15/16-mer PNA probes to detect the HER2 mRNA. The performance of these probes to detect the HER2 target was evaluated by fluorescence imaging and fluorescence bead assays. The PNA probes have sufficiently discriminated between the wild type HER2 target and the mutant target with single base mismatches. Furthermore, the probes exhibited excellent linear concentration dependence between 0.4 to 400 fmol for the target gene. The results demonstrate potential application of PNAs as diagnostic probes with high specificity for quantitative measurements of amplifications or over-expressions of oncogenes.

Facilitating mismatch discrimination by surface-affixed PNA probes via ionic regulation

There has been a search for alternative nucleic acids that can be more effectively used in nucleic acid detection technologies compared to the DNA probes. Peptide nucleic acid (PNA), which contains a non-ionic peptidic backbone, offers such possibilities since it is nuclease-resistant, it binds to DNA with high affinity, and it can be readily self-assembled onto solid substrates, e.g., gold(111), with a molecular backbone orientation away from the substrate. Although application of PNA as a sensor probe has been exemplified, so far there is little or no account of the ionic modulation of single base mismatch discrimination capacity of surface-tethered PNA probes. Herein, we report "on-surface" melting temperatures of PNA-DNA duplexes formed on gold(111) surface, as obtained from fluorescence measurements. We show that surface-tethered PNA forms a stabler duplex than DNA, and is more effective in single base mismatch discrimination than DNA. Importantly, although PNA backbone is non-ionic, variation in the ionic components in hybridization buffer, i.e., varying concentration of monovalent sodium ion, and the nature of anion and the cation, exhibits clear effects on the mismatch discrimination capacity of PNA probes. In general, with decreasing cation concentration, PNA-DNA duplexes are stabilized and mismatch discrimination capacity of the PNA probes is enhanced. The stabilizing/destabilizing effects of anions are found to follow the Hofmeister series, emphasizing the importance of hydrophobic interaction between nucleobases for stability of the PNA-DNA duplexes. Interestingly, the nature of ionic dependence of "on-surface" mismatch detection ability of PNA probes differs significantly from the "solution" behavior of these probes.

Evaluating the effect of ionic strength on duplex stability for PNA having negatively or positively charged side chains

The enhanced thermodynamic stability of PNA:DNA and PNA:RNA duplexes compared with DNA:DNA and DNA:RNA duplexes has been attributed in part to the lack of electrostatic repulsion between the uncharged PNA backbone and negatively charged DNA or RNA backbone. However, there are no previously reported studies that systematically evaluate the effect of ionic strength on duplex stability for PNA having a charged backbone. Here we investigate the role of charge repulsion in PNA binding by synthesizing PNA strands having negatively or positively charged side chains, then measuring their duplex stability with DNA or RNA at varying salt concentrations. At low salt concentrations, positively charged PNA binds more strongly to DNA and RNA than does negatively charged PNA. However, at medium to high salt concentrations, this trend is reversed, and negatively charged PNA shows higher affinity for DNA and RNA than does positively charged PNA. These results show that charge screening by counterions in solution enables negatively charged side chains to be incorporated into the PNA backbone without reducing duplex stability with DNA and RNA. This research provides new insight into the role of electrostatics in PNA binding, and demonstrates that introduction of negatively charged side chains is not significantly detrimental to PNA binding affinity at physiological ionic strength. The ability to incorporate negative charge without sacrificing binding affinity is anticipated to enable the development of PNA therapeutics that take advantage of both the inherent benefits of PNA and the multitude of charge-based delivery technologies currently being developed for DNA and RNA.

Peptide nucleic acids (PNAs) patterning by an automated microarray synthesis system through photolithography

Peptide nucleic acids (PNA) microarray assembled with hundreds of unique PNA oligomers has been regarded as a new and mighty competitor of DNA chip in gene analyzing. However, PNA microarray is still a luxury art due to the difficult and laborious chemical synthesis. Herein, we have developed a fully-automated synthesizer for PNA microarray through photolithography. A preactivation mixer was designed and integrated into the synthesizer in order to get rid of the annoying manual process and increase the coupling efficiency of PNA monomers. The PNA patterning model was carried out to check the performance of the automated synthesizer, revealing that an exposure time of 3 min was sufficient for the complete removal of o-nitroveratryloxycarbonyl (NVOC) groups from the synthetic sites with the help of photosensitizer isopropylthioxanthone and the stepwise yield was measured to be about 98.0%, which is comparable with that from conventional fluorenyl-methyloxycarbonyl (FMOC) chemistry. Those results have definitely demonstrated the possibility and capability of this fully-automated synthesizer to fabricate high-quality PNA microarrays.

Role of SbmA in the uptake of peptide nucleic acid (PNA)-peptide conjugates in E. coli

Antisense PNA oligomers targeting essential genes (acpP or ftsZ) and conjugated to the delivery peptide L((KFF)(3)K) show complete growth inhibition of wild type E. coli strain (MG1655) with submicromolar MIC. In this study we show that resistant mutants generated against such PNA-peptide conjugates had disruptions in the region of sbmA, a gene encoding an inner membrane peptide transporter. The wild type sensitivity to the PNA conjugates was re-established in the resistance mutants by complementation with sbmA. Furthermore, deletion of sbmA in E. coli AS19, a strain that is sensitive to unmodified PNA, resulted in resistance to PNA. Finally, PNA conjugated with the corresponding non-biological H-D((KFF)(3)K) peptide retained antibacterial activity in sbmA deletion strains, whereas the same conjugate with a protease-sensitive linker did not. These results clearly identify SbmA as a carrier of naked PNA over the inner bacterial membrane and thereby infer that the peptide is transporting the PNA conjugates over the outer membrane. Strains lacking SbmA were used to screen novel peptide-PNA carriers that were SbmA-independent. Four such PNA-peptide conjugates, H-D((KFF)(3)K), H-(RFR)(4)-Ahx-βAla, H-(R-Ahx-R)(4)-Ahx-βAla, and H-(R-Ahx)(6)-βAla, were identified that utilize an alternative uptake mechanism but retain their antimicrobial potency. In addition SbmA is the first protein identified to recognize PNA.

SPPS resins impact the PNA-syntheses' improvement

The personalized medicine, also documented as "individualized medicine", is an effective and therapeutic approach. It is designed to treat the disease of the individual patient whose precise differential gene expression profile is well known. The trend in the biomedical and biophysical research shows important consequences for the pharmaceutical drug and diagnostics research. It requires a high variability in the design and safety of target-specific pharmacologically active molecules and diagnostic components for imaging of metabolic processes. A key technology which may fulfill the highest demands during synthesis of these individual drugs and diagnostics is the solid phase synthesis which is congenial to automated manufacturing. Additionally the choice of tools like resins and reagents is pivotal to synthesize drugs and diagnostics in high quality and yields. Here we demonstrate the solid phase synthesis effects dependent on the choice of resin and of the deprotection agent.

Self-assembled antibody multimers through peptide nucleic acid conjugation

With the recent clinical success of bispecific antibodies, a strategy to rapidly synthesize and evaluate bispecific or higher order multispecific molecules could facilitate the discovery of new therapeutic agents. Here, we show that unnatural amino acids (UAAs) with orthogonal chemical reactivity can be used to generate site-specific antibody-oligonucleotide conjugates. These constructs can then be self-assembled into multimeric complexes with defined composition, valency, and geometry. With this approach, we generated potent bispecific antibodies that recruit cytotoxic T lymphocytes to Her2 and CD20 positive cancer cells, as well as multimeric antibody fragments with enhanced activity. This strategy should accelerate the synthesis and in vitro characterization of antibody constructs with unique specificities and molecular architectures.

Cyclopentane-peptide nucleic acids for qualitative, quantitative, and repetitive detection of nucleic acids

We report the development of chemically modified peptide nucleic acids (PNAs) as probes for qualitative and quantitative detection of DNA. The remarkable stability of PNAs toward enzymatic degradation makes this class of molecules ideal to develop as part of a diagnostic device that can be used outside of a laboratory setting. Using an enzyme-linked reporter assay, we demonstrate that excellent levels of detection and accuracy for anthrax DNA can be achieved using PNA probes with suitable chemical components designed into the probe. In addition, we report on DNA-templated cross-linking of PNA probes as a way to preserve genetic information for repetitive and subsequent analysis. This report is the first detailed examination of the qualitative and quantitative properties of chemically modified PNA for nucleic acid detection and provides a platform for studying and optimizing PNA probes prior to incorporation into new technological platforms.

Peptide nucleic acid molecular beacons for the detection of PCR amplicons in droplet-based microfluidic devices

The use of droplet-based microfluidics and peptide nucleic acid molecular beacons for the detection of polymerase chain reaction (PCR)-amplified DNA sequences within nanoliter-sized droplets is described in this work. The nanomolar-attomolar detection capabilities of the method were preliminarily tested by targeting two different single-stranded DNA sequences from the genetically modified Roundup Ready soybean and the Olea europaea genomes and detecting the fluorescence generated by peptide nucleic acid molecular beacons with fluorescence microscopy. Furthermore, the detection of 10 nM solutions of PCR amplicon of DNA extracted from leaves of O. europaea L. encapsulated in nanoliter-sized droplets was performed to demonstrate that peptide nucleic acid molecular beacons can discriminate O. europaea L. cultivar species carrying different single-nucleotide polymorphisms.

Di-heterometalation of thiol-functionalized peptide nucleic acids

As a proof-of-principle, two hetero-bimetallic PNA oligomers containing a ruthenium(II) polypyridyl and a cyclopentadienyl manganese tricarbonyl complex have been prepared by serial combination of solid-phase peptide coupling and in-solution thiol chemistry. Solid-phase N-terminus attachment of Ru(II)-polypyridyl carboxylic acid derivative, C1, onto the thiol-functionalized PNA backbone (H-a-a-g-t-c-t-g-c-linker-cys-NH 2) has been performed by standard peptide coupling method. As two parallel approaches, the strong affinity of thiols for maleimide and haloacetyl group has been exploited for subsequent post-SPPS addition of cymantrene-based organometallic cores, C2 and C3. Michael-like addition and thioether ligation of thiol functionalized PNA1 (H-gly-a-a-g-t-c-t-g-c-linker-cys-NH 2) and PNA2 (C1-a-a-g-t-c-t-g-c-linker-cys-NH 2) to cymantrene maleimide and chloroacetyl derivatives, C2 and C3, respectively, has been performed. The synthesized ruthenium(II)-cymantrenyl PNA oligomers have been characterized by mass spectrometry (ESI-MS) and IR spectroscopy. The distinct Mn-CO vibrational IR stretches, between 1,924-2,074 cm (-1) , have been used as markers to confirm the presence of cymantrenyl units in the PNA sequences and the purity of the HPLC-purified PNA thioethers assessed using LC-MS.

Triple-helical recognition of RNA using 2-aminopyridine-modified PNA at physiologically relevant conditions

Peptide nucleic acids containing thymidine and 2-aminopyridine (M) nucleobases form stable and sequence-selective triple helices with double-stranded RNA at physiologically relevant conditions. The M-modified PNA showed unique RNA selectivity by having two orders of magnitude higher affinity for the double-stranded RNAs than for the same DNA sequences.

Heterogeneous nanoclusters assembled by PNA-templated double-stranded DNA

Heterogeneous nanoclusters with trimeric and core-shell architectures containing nanoparticles of different size and composition have been fabricated via site-specific PNA-"invasion" of DNA double helix. This novel strategy facilitates the incorporation of double-stranded DNA into the nanoparticle assembly design.

Effects of polyethylene glycol on DNA adsorption and hybridization on gold nanoparticles and graphene oxide

Understanding the interface between DNA and nanomaterials is crucial for rational design and optimization of biosensors and drug delivery systems. For detection and delivery into cells, where high concentrations of cellular proteins are present, another layer of complexity is added. In this context, we employ polyethylene glycol (PEG) as a model polymer to mimic the excluded volume effect of cellular proteins and to test its effects on DNA adsorption and hybridization on gold nanoparticles (AuNPs) and graphene oxide (GO), both of which show great promise for designing intracellular biosensors and drug delivery systems. We show that PEG 20000 (e.g., 4%) accelerates DNA hybridization to DNA-functionalized AuNPs by 50-100%, but this enhanced hybridization kinetics has not been observed with free DNA. Therefore, this rate enhancement is attributed to the surface blocking effect by PEG instead of the macromolecular crowding effect. On the other hand, DNA adsorption on citrate-capped AuNP surfaces is impeded even in the presence of a trace level (i.e., parts per billion) of PEG, confirming PEG competes with DNA for surface binding sites. Additional insights have been obtained by studying the adsorption of a thiolated DNA and a peptide nucleic acid. In these cases, the steric effects of PEG to impede adsorption are observed. Similar observations have also been made with GO. Therefore, PEG may be used as an effective blocking agent for both hydrophilic AuNP and for GO that also contains hydrophobic domains.