Dye-Assembled Upconversion Nanocomposite for Luminescence Ratiometric in Vivo Bioimaging of Copper Ions

Advancements in diagnostic approaches for Wilson's disease

Wilson's disease (WD) is a genetic disorder that results in excessive copper build-up in tissues, causing significant liver and neurological damage. Early and accurate diagnosis is crucial for effective management and treatment. Traditional diagnostic methods, including serum ceruloplasmin and urinary copper level monitoring, liver biopsy and genetic testing, are limited by sensitivity, specificity, invasiveness, and accessibility. Recent advances in diagnostic technologies offer new hope for more accurate, rapid, and non-invasive detection of WD. In this regard, nanotechnology-driven formulations hold significant promise for both the early diagnosis and treatment of WD. Through the innovative use of advanced nanomaterials, researchers are developing more effective therapeutic options and highly sensitive diagnostic tools, potentially transforming the management and prognosis of this genetic disorder. This review provides a comprehensive analysis of recent advancements in WD diagnostics, focusing on research published since 2016. It explores the development and application of novel biomarkers, advanced imaging modalities, innovative biosensors, and emerging nanotechnology-based approaches. By integrating these cutting-edge methodologies, the review highlights their potential to enhance early and accurate detection of WD, addressing current diagnostic challenges and improving clinical outcomes.

Intensive and Persistent Chemiluminescence from Orderly Arranged Ligands within Metal-Organic Frameworks for Inflammation Imaging

Chemiluminescence offers ultrasensitive imaging for the diagnosis of a variety of diseases by removing the interference from excitation light sources. Here, we prepared two chemiluminescent metal-organic frameworks (Mn-ADA and Zn-ADA) by using (2E,2'E)-3,3'-(anthracene-9,10-diyl)diacrylic acid (ADA) as a ligand. In Mn-ADA and Zn-ADA, the Mn atoms and Zn atoms are six-coordinated and eight-coordinated, respectively, and their frameworks are different in spatial structure. Due to the orderly arrangement of the fluorescence ligands and one-dimensional channel control of the diffusion of the reactant, Mn-ADA exhibits superstrong intensity and persistent chemiluminescence compared to ADA. The intensity of Mn-ADA is 43 times higher, and the lifetime is two times longer than that of ADA. Furthermore, different coordination also causes the chemiluminescence intensity of Mn-ADA to be stronger than that of Zn-ADA. It is established that Mn-ADA can detect H2O2 and image inflammation in mice without the excitation light. This methodology demonstrates the potential of metal-organic frameworks (MOFs) to enhance chemiluminescence and offers a new avenue for the development of MOF materials intended for biomedical application.

Selective and sensitive CQD-based sensing platform for Cu2+ detection in Wilson's disease

Excessive Cu2+ intake can cause neurological disorders (e.g. Wilson's disease) and adversely affect the gastrointestinal, liver, and kidney organs. The presence of Cu2+ is strongly linked to the emergence and progression of Wilson's disease (WD), and accurately measuring the amount of copper is a crucial step in diagnosing WD at an early stage in a clinical setting. In this work, CQDs were fabricated through a facile technique as a novel fluorescence-based sensing platform for detecting Cu(II) in aqueous solutions, and in the serum samples of healthy and affected individuals by WD. The CQDs interact with Cu(II) ions to produce Turn-on and Turn-off states at nano-molar and micro-molar levels, respectively, with LODs of 0.001 µM and 1 µM. In fact, the Cu2+ ions can act like a bridge between two CQDs by which the charge and electron transfer between the CQDs may increase, possibly can have significant effects on the spectroscopic features of the CQDs. To the best of our knowledge, this is the first reported research that can detect Cu(II) at low levels using two different complexation states, with promising results in testing serum. The potential of the sensor to detect Cu(II) was tested on serum samples from healthy and affected individuals by WD, and compared to results obtained by ICP-OES. Astonishingly, the results showed an excellent correlation between the measured Cu(II) levels using the proposed technique and ICP-OES, indicating the high potential of the fluorimetric CQD-based probe for Cu(II) detection. The accuracy, sensitivity, selectivity, high precision, accuracy, and applicability of the probe toward Cu(II) ions make it a potential diagnostic tool for Wilson's disease in a clinical setting.

Ratiometric upconversion luminescence nanoprobes from construction to sensing, imaging, and phototherapeutics

In terms of the combined advantages of upconversion luminescence (UCL) properties and dual-signal ratiometric outputs toward specific targets, the ratiometric UCL nanoprobes exhibit significant applications. This review summarizes and discusses the recent advances in ratiometric UCL nanoprobes, mainly including the construction of nanoprobe systems for sensing, imaging, and phototherapeutics. First, the construction strategies are introduced, involving different types of nanoprobe systems, construction methods, and ratiometric dual-signal modes. Then, the sensing applications are summarized, involving types of targets, sensing mechanisms, sensing targets, and naked-eye visual detection of UCL colors. Afterward, the phototherapeutic applications are discussed, including bio-toxicity, bio-distribution, biosensing, and bioimaging at the level of living cells and small animals, and biomedicine therapy. Particularly, each section is commented on by discussing the state-of-the-art relevant studies on ratiometric UCL nanoprobe systems. Moreover, the current status, challenges, and perspectives in the forthcoming studies are discussed. This review facilitates the exploration of functionally luminescent nanoprobes for excellent sensing, imaging, biomedicine, and multiple applications in significant fields.

Deep-Depth Imaging of Hepatic Ischemia/Reperfusion Injury Using a Carbon Monoxide-Activated Upconversion Luminescence Nanosystem

Exploring a chemical imaging tool for visualizing the endogenous CO biosignaling molecule is of great importance in understanding the pathophysiological functions of CO in complex biological systems. Most of the existing CO fluorescent probes show excitation and emission in the region of ultraviolet and visible light, which are not suitable for application in in vivo deep-depth imaging of CO. Herein, a new near-infrared (NIR) to NIR upconversion luminescence (UCL) nanosystem for in vivo visualization of CO was developed, which possesses the merits of high selectivity and sensitivity, a deep tissue penetration depth, and a high signal-to-noise ratio. In this design, upon interaction with CO, the maxima absorption peak of the nanosystem showed a significant blue shift from 795 nm to 621 nm and triggered a remarkable turn-on NIR UCL signal due to the luminescence resonance energy transfer process. Leveraging this nanosystem, we achieved an NIR UCL visualization of the generation of CO biosignals caused by hypoxic, acute inflammation, or ischemic injury in living cells, zebrafish, and mice. Moreover, the protective effect of CO in zebrafish models of oxygen and glucose deprivation/reperfusion (OGD/R) and mice models of lipopolysaccharide-induced oxidative stress (LOS) and hepatic ischemia/reperfusion (HI/R) was also further verified. Therefore, this work discloses that the nanosystem not only serves as a promising nanoplatform to study biological signaling pathways of CO in pathophysiological events, but may also provide a powerful tool for HI/R injury diagnosis.

Recent Advances of Optical Sensors for Copper Ion Detection

A trace element copper (Cu2+) ion is the third most plentiful metal ion that necessary for all living organisms and playing a critical role in several processes. Nonetheless, according to cellular needs, deficient or excess Cu2+ ion cause various diseases. For all these reasons, optical sensors have been focused rapid Cu2+ ion detection in real-time with high selectivity and sensitivity. Optical sensors can measure fluorescence in the refractive index-adsorption from the relationships between light and matter. They have gained great attention in recent years due to the excellent advantages of simple and naked eye recognition, real-time detection, low cost, high specificity against analytes, a quick response, and the need for less complex equipment in analysis. This review aims to show the significance of Cu2+ ion detection and electively current trends in optical sensors. The integration of optical sensors with different systems, such as microfluidic systems, is mentioned, and their latest studies in medical and environmental applications also are depicted. Conclusions and future perspectives on these advances is added at the end of the review.

Micellar Ratiometric Fluorescent Blood pH Probe Based on Triplet-Sensitized Upconversion and Energy-Transfer Behaviors

The measurement of pH is greatly significant in monitoring physiological and biochemical states. In this work, a novel micellar ratiometric fluorescent probe featuring sophisticated energy-transfer (ET) behaviors with p-nitrophenol (PNP) as the energy acceptor and a triplet-triplet annihilation upconversion (TTA-UC) system as the energy donor was designed. The pH-induced molecular configuration of PNP determined the process for the transfer of energy from TTA-UC to PNP. The introduction of the TTA-UC system enabled probe excitation under a long wavelength and afforded a ratiometric signal for pH detection with excellent reliability over diverse interfering factors. This TTA-UC/ET pH probe demonstrated a high sensitivity to hydronium below nanomolar concentrations and an excellent anti-interference ability in serum samples, which provided a novel significant strategy for rapid and accurate detection of blood pH in vitro.

Significantly Promoting the Photogenerated Charge Separation by Introducing an Oxygen Vacancy Regulation Strategy on the FeNiOOH Co-Catalyst

Semiconductor/co-catalyst coupling is considered as a promising strategy to enhance the photoelectrochemical (PEC) conversion efficiency. Unfortunately, this model system is faced with a serious interface recombination problem, which limits the further improvement of PEC performances. Here, a FeNiOOH co-catalyst with abundant oxygen vacancies on BiVO₄ is fabricated through simple and economical NaBH₄ reduction to accelerate hole transfer and achieve efficient electron-hole pair separation. The photocurrent of the BV (BiVO₄ )/Vo-FeNiOOH system is more than four times that of pure BV. Importantly, the charge transfer kinetics and charge carrier recombination process are studied by scanning photoelectrochemical microscopy and intensity modulated photocurrent spectroscopy in detail. In addition, the oxygen vacancy regulation proposed is also applied successfully to other semiconductors (Fe₂ O₃ ), demonstrating the applicability of this strategy.

Hybrid Mesoporous MnO2-Upconversion Nanoparticles for Image-Guided Lung Cancer Spinal Metastasis Therapy

Upconversion nanoparticles (UCNPs) and MnO₂ composite materials have broad prospects in biological applications due to their near-infrared (NIR) imaging capability and tumor microenvironment-responsive features. Nevertheless, the synthesis of such composite nanoplatforms still faces many hurdles such as redundant processing and uneven coatings. Here, we explored a simple, rapid, and universal method for precisely controlled coating of mesoporous MnO₂ (mMnO₂) using poly(ethylene imine) as a reducing agent and potassium permanganate as a manganese source. Using this strategy, a mMnO₂ shell was successfully coated on UCNPs. We further modified the mMnO₂-coated UCNPs (UCNP@mMnO₂) with a photosensitizer (Ce6), cisplatin drug (DSP), and tumor targeting pentapeptide (TFA) to obtain a nanoplatform UCNP/Ce6@mMnO₂/DSP-TFA for treating spinal metastasis of nonsmall cell lung cancer (NSCLC-SM). The utilization of both upconversion and downconversion luminescence of UCNPs with different NIR wavelengths can avoid the simultaneous initiation of NIR-II in vivo imaging and tumor photodynamic therapy, thus reducing damage to normal tissues. This platform achieved a high synergistic effect of photodynamic therapy and chemotherapy. This leads to beneficial antitumor effects on the therapy of NSCLC-SM.

A novel polypeptide-modified fluorescent gold nanoclusters for copper ion detection

Biomolecule-functionalized fluorescent gold nanocluster (AuNCs) have attracted a lot of attention due to good biocompatibility, stable physicochemical properties and considerable cost advantages. Inappropriate concentration of Cu²⁺ may cause a variety of diseases. In this study, AuNCs were synthesized in alkaline aqueous solution using bovine serum albumin (BSA) as a template. And then, the peptide CCYWDAHRDY was coupled to AuNCs. Furthermore, the fluorescence of synthesized CCYWDAHRDY-AuNCs response to Cu²⁺ was evaluated. As the results shown that the CCYWDAHRDY-AuNCs can sensitively detect Cu²⁺. After adding Cu²⁺ to the probe system, the fluorescence of the CCYWDAHRDY-AuNCs was quenched. The detection conditions were at pH 6 and 30 °C for 10 min, the linear relationship between Cu²⁺ concentration and fluorescence intensity were good in the range of 0.1 ~ 4.2 μmol/L. The regression equation was y = − 105.9x + 693.68, the linear correlation coefficient is 0.997, and the minimum detection limit was 52 nmol/L.

A novel dual-emission fluorescent probe for ratiometric and visual detection of Cu2+ ions and Ag+ ions

In this work, the biomolecule glutathione was used to prepare cyan fluorescent carbon dots (GSH@CDs) by a hydrothermal method. The GSH@CDs were adopted as the scaffolds to synthesize fluorescent gold nanoclusters (GSH@CDs-Au NCs) with two independent emission peaks at 430 nm and 700 nm. A fluorescent method for the Cu²⁺ and Ag⁺ ion assay was established based on the fluorescence quenching or enhancement at 700 nm of GSH@CDs-Au NCs. The fluorescent test strips were successfully prepared for visual detection of Cu²⁺ ions and Ag⁺ ions based on GSH@CDs-Au NCs. In addition, GSH@CDs-Au NCs were found to possess well peroxidase-like activity.

Advances in fluorescence sensing enabled by lanthanide-doped upconversion nanophosphors

Lanthanide-doped upconversion nanoparticles (UCNPs), characterized by converting low-energy excitation to high-energy emission, have attracted considerable interest due to their inherent advantages of large anti-Stokes shifts, sharp and narrow multicolor emissions, negligible autofluorescence background interference, and excellent chemical- and photo-stability. These features make them promising luminophores for sensing applications. In this review, we give a comprehensive overview of lanthanide-doped upconversion nanophosphors including the fundamental principle for the construction of UCNPs with efficient upconversion luminescence (UCL), followed by state-of-the-art strategies for the synthesis and surface modification of UCNPs, and finally describing current advances in the sensing application of upconversion-based probes for the quantitative analysis of various analytes including pH, ions, molecules, bacteria, reactive species, temperature, and pressure. In addition, emerging sensing applications like photodetection, velocimetry, electromagnetic field, and voltage sensing are highlighted.

Advanced NIR ratiometric probes for intravital biomedical imaging

Near-infrared (NIR) fluorescence imaging technology (NIR-I region, 650-950 nm and NIR-II region, 1000-1700 nm), with deeper tissue penetration and less disturbance from auto-fluorescence than that in visible region (400-650 nm), is playing a more and more extensive role in the field of biomedical imaging. With the development of precise medicine, intelligent NIR fluorescent probes have been meticulously designed to provide more sensitive, specific and accurate feedback on detection. Especially, recently developed ratiometric fluorescent probes have been devoted to quantify physiological and pathological parameters with a combination of responsive fluorescence changes and self-calibration. Herein, we systemically introduced the construction strategies of NIR ratiometric fluorescent probes and their applications in biological imagingin vivo, such as molecular detection, pH and temperature measurement, drug delivery monitoring and treatment evaluation. We further summarized possible optimization on the design of ratiometric probes for quantitative analysis with NIR fluorescence, and prospected the broader optical applications of ratiometric probes in life science and clinical translation.

Nanomedicine against Alzheimer's and Parkinson's Disease

Alzheimer's and Parkinson's are the two most rampant neurodegenerative disorders worldwide. Existing treatments have a limited effect on the pathophysiology but are unable to fully arrest the progression of the disease. This is due to the inability of these therapeutic molecules to efficiently cross the blood-brain barrier. We discuss how nanotechnology has enabled researchers to develop novel and efficient nano-therapeutics against these diseases. The development of nanotized drug delivery systems has permitted an efficient, site-targeted, and controlled release of drugs in the brain, thereby presenting a revolutionary therapeutic approach. Nanoparticles are also being thoroughly studied and exploited for their role in the efficient and precise diagnosis of neurodegenerative conditions. We summarize the role of different nano-carriers and RNAi-conjugated nanoparticle-based therapeutics for their efficacy in pre-clinical studies. We also discuss the challenges underlying the use of nanomedicine with a focus on their route of administration, concentration, metabolism, and any toxic effects for successful therapeutics in these diseases.

Quantitative Assessment of Copper(II) in Wilson's Disease Based on Photoacoustic Imaging and Ratiometric Surface-Enhanced Raman Scattering

Cu²⁺ is closely related to the occurrence and development of Wilson's disease (WD), and quantitative detection of various copper indicators (especially liver Cu² and urinary Cu²⁺) is the key step for the early diagnosis of WD in the clinic. However, the clinic Cu²⁺ detection approach was mainly based on testing the liver tissue through combined invasive liver biopsy and the ICP-MS method, which is painful for the patient and limited in determining WD status in real-time. Herein, we rationally designed a type of Cu²⁺-activated nanoprobe based on nanogapped gold nanoparticles (AuNNP) and poly(N-isopropylacrylamide) (PNIPAM) to simultaneously quantify the liver Cu²⁺ content and urinary Cu²⁺ in WD by photoacoustic (PA) imaging and ratiometric surface-enhanced Raman scattering (SERS), respectively. In the nanoprobe, one Raman molecule of 2-naphthylthiol (NAT) was placed in the nanogap of AuNNP. PNIPAM and the other Raman molecule mercaptobenzonitrile (MBN) were coated on the AuNNP surface, named AuNNP-NAT@MBN/PNIPAM. Cu²⁺ can efficiently coordinate with the chelator PNIPAM and lead to aggregation of the nanoprobe, resulting in the absorption red-shift and increased PA performance of the nanoprobe in the NIR-II window. Meanwhile, the SERS signal at 2223 cm^-1 of MBN is amplified, while the SERS signal at 1378 cm^-1 of NAT remains stable, generating a ratiometric SERS I₂₂₂₃/I₁₃₇₈ signal. Both NIR-II PA_1250 nm and SERS I₂₂₂₃/I₁₃₇₈ signals of the nanoprobe show a linear relationship with the concentration of Cu²⁺. The nanoprobe was successfully applied for in vivo quantitative detection of liver Cu²⁺ of WD mice through NIR-II PA imaging and accurate quantification of urinary Cu²⁺ of WD patients by ratiometric SERS. We anticipate that the activatable nanoprobe might be applied for assisting an early, precise diagnosis of WD in the clinic in the future.

Cyanine-modified near-infrared upconversion nanoprobe for ratiometric sensing of N2H4 in living cells

Although being as an important chemical material in industry, hydrazine (N₂H₄) is highly toxic to the humans and animals. The development of sensitive methods for the detection of hydrazine is meaningful. Herein, we develop a new organic-inorganic hybrid nanoprobe for the detection of N₂H₄ based on luminescent resonance energy transfer (LRET) process. The nanoprobe contains N₂H₄-responsive NIR cyanine dye (CQM1) and α-cyclodextrin (CD) anchored on the surface of lanthanide-doped upconversion nanophosphors (UCNPs). In the presence of hydrazine, the hybrid materials (CQM1-UCNPs) showed the a large ratiometric luminescent signal change with high sensitivity and selectivity. More importantly, by taking advantage of ratiometric Upconversion luminescent (UCL) signal and the features of NIR emission/excitation, the nanoprobe was successfully applied for visualization of hydrazine in living cells for the first time.

Opportunities for Persistent Luminescent Nanoparticles in Luminescence Imaging of Biological Systems and Photodynamic Therapy

The use of luminescence in biological systems allows us to diagnose diseases and understand cellular processes. Persistent luminescent materials have emerged as an attractive system for application in luminescence imaging of biological systems; the afterglow emission grants background-free luminescence imaging, there is no need for continuous excitation to avoid tissue and cell damage due to the continuous light exposure, and they also circumvent the depth penetration issue caused by excitation in the UV-Vis. This review aims to provide a background in luminescence imaging of biological systems, persistent luminescence, and synthetic methods for obtaining persistent luminescent materials, and discuss selected examples of recent literature on the applications of persistent luminescent materials in luminescence imaging of biological systems and photodynamic therapy. Finally, the challenges and future directions, pointing to the development of compounds capable of executing multiple functions and light in regions where tissues and cells have low absorption, will be discussed.

Activatable Photodynamic Therapy with Therapeutic Effect Prediction Based on a Self-correction Upconversion Nanoprobe

Though emerging as a promising therapeutic approach for cancers, the crucial challenge for photodynamic therapy (PDT) is activatable phototoxicity for selective cancer cell destruction with low "off-target" damage and simultaneous therapeutic effect prediction. Here, we design an upconversion nanoprobe for intracellular cathepsin B (CaB)-responsive PDT with in situ self-corrected therapeutic effect prediction. The upconversion nanoprobe is composed of multishelled upconversion nanoparticles (UCNPs) NaYF₄:Gd@NaYF₄:Er,Yb@NaYF₄:Nd,Yb, which covalently modified with an antenna molecule 800CW for UCNPs luminance enhancement under NIR irradiation, photosensitizer Rose Bengal (RB) for PDT, Cy3 for therapeutic effect prediction, and CaB substrate peptide labeled with a QSY7 quencher. The energy of UCNPs emission at 540 nm is transferred to Cy3/RB and eventually quenched by QSY7 via two continuous luminance resonance energy transfer processes from interior UCNPs to its surface-extended QSY7. The intracellular CaB specifically cleaves peptide to release QSY7, which correspondingly activates RB with reactive oxygen species (ROS) generation for PDT and recovers Cy3 luminance for CaB imaging. UCNPs emission at 540 nm remains unchanged during the peptide cleavage process, which is served as an internal standard for Cy3 luminance correction, and the fluorescence intensity ratio of Cy3 over UCNPs (FI583/FI540) is measured for self-corrected therapeutic effect prediction. The proposed self-corrected upconversion nanoprobe implies significant potential in precise tumor therapy.

Near-Infrared Lanthanide-Doped Nanoparticles for a Low Interference Lateral Flow Immunoassay Test

The lateral flow immunoassay test (LFT), as a method of a point of care test, is widely used in disease diagnosis, food security, and environment observation due to its portability and testing rapidity. A fluorescence lateral flow immunoassay was developed recently to enhance the sensitivity and accuracy of the LFT. However, for most fluorescence reporters, their emission and excitation wavelengths are located in the ultraviolet or visible region. Serum or whole blood significantly absorbs and scatters light of this region, and this will result in background signal interference. In this study, we replace traditional fluorescence reporters with near-infrared lanthanide-doped nanoparticles (NIR-RENPs) to establish a NIR-LFT platform. Blood and other biological samples scatter and absorb less near-infrared light than visible light, and the autofluorescence of biological samples is rarely located in this region. Therefore, using NIR light as a signal can diminish the interference of background noise and suffer from less signal attenuation. In addition, compared with commonly used NIR organic dye, NIR-RENPs have better stability. It is promising that lateral flow immunoassays based on NIR lanthanide-doped nanoparticles are able to acquire a lower detection limit and better accuracy, and they are more suitable for application in commercial settings.

A coumarin-containing Schiff base fluorescent probe with AIE effect for the copper(ii) ion

A novel coumarin-derived Cu²⁺-selective Schiff base fluorescent “turn-off” chemosensor CTPE was successfully obtained, which showed an AIE effect. It could identify Cu²⁺ by quenching its fluorescence. The lower limit of detection was 0.36 μM. CTPE can act as a highly selective and sensitive fluorescence probe for detecting Cu²⁺.

A novel coumarin-derived Schiff base fluorescent “turn-off” chemosensor with AIE effect showed selectivity towards Cu²⁺. The recognition mechanism is presented.

Optical Detection of Intracellular Quantities Using Nanoscale Technologies

Optical probes that can be used to measure certain quantities with subcellular resolution give us access to a new level of information at which physics, chemistry, life sciences, and medicine become strongly intertwined. The emergence of these new technologies is owed to great advances in the physical sciences. However, evaluating and improving these methods to new standards requires a joint effort with life sciences and clinical practice. In this Account, we give an overview of the probes that have been developed for measuring a few highly relevant parameters at the subcellular scale: temperature, pH, oxygen, free radicals, inorganic ions, genetic material, and biomarkers. Luminescent probes are available in many varieties, which can be used for measuring temperature, pH, and oxygen. Since they are influenced by virtually any metabolic process in the healthy or diseased cell, these quantities are extremely useful to understand intracellular processes. Probes for them can roughly be divided into molecular dyes with a parameter dependent fluorescence or phosphorescence and nanoparticle platforms. Nanoparticle probes can provide enhanced photostability, measurement quality, and potential for multiple functionalities. Embedding into coatings can improve biocompatibility or prevent nonspecific interactions between the probe and the cellular environment. These qualities need to be matched however with good uptake properties, colloidal properties and eventually intracellular targeting to optimize their practical applicability. Inorganic ions constitute a broad class of compounds or elements, some of which play specific roles in signaling, while others are toxic. Their detection is often difficult due to the cross-talk with similar ions, as well as other parameters. The detection of free radicals, DNA, and biomarkers at extremely low levels has significant potential for biomedical applications. Their presence is linked more directly to physiological and clinical manifestations. Since existing methods for free radical detection are generally poor in sensitivity and spatiotemporal resolution, new reliable methods that are generally applicable can contribute greatly to advancing this topic in biology. Optical methods that detect DNA or RNA and protein biomarkers exist for intracellular applications, but are mostly relevant for the development of rapid point-of-care sample testing. To elucidate the inner workings of cells, focused multidisciplinary research is required to define the validity and limitations of a nanoparticle probe, in both physical and biological terms. Multifunctional platforms and those that are easily made compatible with conventional research equipment have an edge over other techniques in growing the body of research evidencing their versatility.